The Trouvelot astronomical drawings manual

J-NRLF

HG

S71

_

Astronomical Drawings

MANUAL

&£

Do

THE

TROUVELOT

ASTRONOMICAL DRAWINGS

MANUAL

THE

TROUVELOT

Astronomical Drawings

MANUAL

BY

E. L. TROUVELOT,

FORMERLY CONNECTED WITH THE OBSERVATORY OF HARVARD COLLEGE ; FELLOW OF THE
AMERICAN ACADEMY OF ARTS AND SCIENCES, AND MEMBER OF THE SELENO-
GRAPHICAL SOCIETY OF GREAT BRITAIN ; IN CHARGE OF A
GOVERNMENT EXPEDITION TO OBSERVE THE
TOTAL SOLAR ECLIPSE OF 1878.

NEW YO/EK

CHARLES SCRpNER'S SONS
18/2

T7

Astron.

ASTRONOMY

COPSTKIGHT BY

CHABLES SCElBNEE'S SONS

1882

NEW YORK

JENKINS A THOMAS, PRINTERS
8 8PBUCE 8TEEET

INTRODUCTION.

DURING a study of the heavens, which has now been continued for
more than fifteen years, I have made a large number of observations
pertaining to physical astronomy, together with many original
drawings representing the most interesting celestial objects and
phenomena.

With a view to making these observations more generally useful,
I was led, some years ago, to prepare, from this collection of draw-
ings, a series of astronomical pictures, which were intended to repre-
sent the celestial phenomena as they appear to a trained eye and to
an experienced draughtsman through the great modern telescopes,
provided with the most delicate instrumental appliances. Over two
years were spent in the preparation of this series, which consisted of
a number of large drawings executed in pastel. In 1876, these draw-
ings were displayed at the United States Centennial Exhibition at
Philadelphia, forming a part of the Massachusetts exhibit, in the
Department of Education and Science.

The drawings forming the present series comprise only a part of
those exhibited at Philadelphia ; but, although fewer in number,
they are quite sufficient to illustrate the principal classes of celestial
objects and phenomena.

While my aim in this work has been to combine scrupulous fidel-
ity and accuracy in the details, I have also endeavored to preserve
the natural elegance and the delicate outlines peculiar to the objects
depicted ; but in this, only a little more than a suggestion is possi-
ble, since no human skill can reproduce upon paper the majestic
beauty and radiance of the celestial objects.

The plates were prepared under my supervision, from the origi-
nal pastel drawings, and great care has been taken to make the
reproduction exact.

The instruments employed in the observations, and in the deline-
ation of the heavenly bodies represented in the series, have varied

670903

VI

in aperture from 6 to 26 inches, according to circumstances, and to
the nature of the object to be studied. The great Washington re-
fractor, kindly placed at my disposal by the late Admiral C. H.
Davis, has contributed to this work, as has also the 26 inch telescope
of the University of Virginia, while in the hands of its celebrated
constructors, Alvan Clark & Sons. The spectroscope used was
made by Alvan Clark & Sons. Attached to it is an excellent dif-
fraction grating, by Mr. L. M. Rutherfurd, to whose kindness I am
indebted for it.

Those unacquainted with the use of optical instruments generally
suppose that all astronomical drawings are obtained by the photo-
graphic process, and are, therefore, comparatively easy to procure ;
but this is not true. Although photography renders valuable assist-
ance to the astronomer in the case of the Sun and Moon, as proved
by the fine photographs of these objects taken by M. Janssen and
Mr. Rutherfurd ; yet, for other subjects, its products are in general
so blurred and indistinct that no details of any great value can be
secured. A well-trained eye alone is capable of seizing the deli-
cate details of structure and of configuration of the heavenly bodies,
which are liable to be affected, and even rendered invisible, by the
slightest changes in our atmosphere.

The method employed to secure correctness in the proportions
of the original drawings is simple, but well adapted to the purpose
in view. It consists in placing a fine reticule, cut on glass, at the
common focus of the objective and the eye-piece, so that in viewing
an object, its telescopic image, appearing projected on the reticule,
can be drawn very accurately on a sheet of paper ruled with corre-
sponding squares. For a series of such reticules I am indebted to
the kindness of Professor William A. Rogers, of the Harvard College
Observatory.

The drawings representing telescopic views are inverted, as they
appear in a refracting telescope — the South being upward, the North
downward, the East on the right, and the West on the left. The
Comet, the Milky- Way, the Eclipse of the Moon, the Aurora Borea-
lis, the Zodiacal Light and the Meteors are represented as seen
directly in the sky with the naked eye. The Comet was, however,
drawn with the aid of the telescope, without which the delicate
structure shown in the drawing would not have been visible.

The plate representing the November Meteors, or so-called
" Leonids," may be called an ideal view, since the shooting stars
delineated, were not observed at the same moment of time, but dur-
ing the same night. Over three thousand Meteors were observed

Vll

between midnight and five o'clock in the morning of the day on
which this shower occurred ; a dozen being sometimes in sight at
the same instant. The paths of the Meteors, whether curved, wavy,
or crooked, and also their delicate colors, are in all cases depiqted
as they were actually observed.

In the Manual, I have endeavored to present a general outline
of what is known, or supposed, on the different subjects and phe-
nomena illustrated in the series. The statements made are derived
either from the best authorities on physical astronomy, or from my
original observations, which are, for the most part, yet unpublished.

The figures in the Manual relating to distance, size, volume,
mass, etc., are not intended to be strictly exact, being only round
numbers, which can, therefore, be more easily remembered.

It gives me pleasure to acknowledge that the experience ac-
quired in making the astronomical drawings published in Volume
VIII. of the Annals of the Harvard College Observatory, while
I was connected with that institution, has been of considerable as-
sistance to me in preparing this work ; although no drawings made
while I was so connected have been used for this series.

E. L. TROUVELOT.

Cambridge, March, 1882.

CONTENTS.

PAGE

INTKODUCTION v

LIST OF PLATES xi

THE SUN.

GENERAL REMARKS ON THE SUN 1

SUN-SPOTS AND VEILED SPOTS 8

SOLAR PROTUBERANCES 18

TOTAL ECLIPSE OF THE SUN 23

THE AURORAL AND ZODIACAL LIGHTS.

THE AURORA BOREALIS 28

THE ZODIACAL LIGHT 36

THE MOON.

THE MOON 42

ECLIPSES OF THE MOON 53

THE PLANETS.

THE PLANETS 57

THE PLANET MARS 61

THE PL YNET JUPITER 73

THE PLANET SATURN. . . 83

X

COMETS AND METEORS.

COMETS 99

SHOOTING-STABS AND METEORS 115

THE STELLAR SYSTEMS.

THE MILKY- WAY OR GALAXY 128

THE STAR-CLUSTERS 137

THE NEBULJS 144

APPENDIX , . . 159

LIST OF PLATES.

Plate I. GROUP OF SUN-SPOTS AND VEILED SPOTS.

Observed June 17, 1875, atjh. jom. A. M.

" II. SOLAR PROTUBERANCES.

Observed May 5,1873, at yh. 4om. A. M.

" III. TOTAL ECLIPSE OF THE SUN.

Observed July 29, i8j8, at Creston, Wyoming Territory.
" IV. AURORA BOREALIS.

As observed March I, 1872, at yh. 2$m. P. M.
" V. THE ZODIACAL LIGHT.

Observed February 20, 1876.

VI. MARE HUMORUM.

From a study made in 187 5.

VII. PARTIAL ECLIPSE OF THE MOON.

Observed October 24, 1874.

VIII. THE PLANET MARS.

Observed September 3, 1877, at I ih. 55™. P. M.
*' IX. THE PLANET JUPITER.

Observed November i, 1880, at yh. jom. P. M.
*' X. THE PLANET SATURN.

Observed November jo, 1874, at $h. $om. P. M.
XI. THE GREAT COMET OF 1881.

Observed on the night of June 25-26, at ih. jom. A. M.

* For Key to the Plates, see Appendix.

Xll

Plate XII. THE NOVEMBER METEORS.

As observed between midnight and 5 o'clock A. M., on the night
of November 13-14, 1868.

XIII. PART OF THE MILKV-WAY.

From a study made during the years 1874, 1875 and 1876.
" XIV. STAR-CLUSTER IN HERCULES.

From a study made in June, 1877.
" XV. THE GREAT NEBULA IN ORION.

From a study made in the years 1875-76.

%* Reproduced from the Original Drawings, by Armstrong 6f Company,
Riverside Press, Cambridge, Mass.

GENERAL REMARKS ON THE SUK

THE Sun, the centre of the system which bears its name, is a self-
luminous sphere, constantly radiating heat and light.

Its apparent diameter, as seen at its mean from the Earth, sub-
tends an angle of 32', or a little over half a degree. A dime, placed
about six feet from the eye, would appear of the same proportions,
and cover the Sun's disk, if projected upon it.

That the diameter of the Sun does not appear larger, is due to
the great distance which separates us from that body. Its distance
from the Earth is no less than 92,000,000 miles. To bridge this
immense gap, would require 11,623 globes like the Earth, placed
side by side, like beads on a string.

The Sun is an enormous sphere whose diameter is over 108 times
the diameter of our globe, or very nearly 860,000 miles. Its radius is
nearly double the distance from the Earth to the Moon. If we sup-
pose, for a moment, the Sun to be hollow, and our globe to be placed
at the centre of this immense spherical shell, not only could our
satellite revolve around us at its mean distance of 238,800 miles, as
now, but another satellite, placed 190,000 miles farther than the
Moon, could freely revolve likewise, without ever coming in con-
tact with the solar envelope.

The circumference of this immense sphere measures 2,800,000
miles. While a steamer, going at the rate of 300 miles a day, would
circumnavigate the Earth in 83 days, it would take, at the same rate,
nearly 25 years to travel around the Sun.

The surface of the Sun is nearly 12,000 times the surface of the
Earth, and its volume is equal to 1,300,000 globes like our own. If
all the known planets and satellites were united in a single mass, 600
such compound masses would be needed to equal the volume of our
luminary.

Although the density of the Sun is only one-quarter that of the
Earth, yet the bulk of this body is so enormous that, to counterpoise
it, no less than 314,760 globes like our Earth would be required.

2 ' THE TROUVELOT

The Sun uniformly revolves around its axis in about 2$% days.
Its equator is inclined 7° 15' to the plane of the ecliptic, the axis of
rotation forming, therefore, an angle of 82° 45' with the same plane.
As the Earth revolves about the Sun in the same direction as that
of the Sun's rotation, the apparent time of this rotation, as seen by
a terrestrial spectator, is prolonged from 25^ days to about 27 days
and 7 hours.

The rotation of the Sun on its axis, like that of the Earth and
the other planets, is direct, or accomplished from West to East. To
an observer on the Earth, looking directly at the Sun, the rotation
of this body is from left to right, or from East to West.

The general appearance of the Sun is that of an intensely lumin-
ous disk, whose limb, or border, is sharply defined on the heavens.
When its telescopic image is projected on a screen, or fixed on
paper by photography, it is noticed that its disk is not uniformly
bright throughout, but is notably more luminous in its central parts.
This phenomenon is not accidental, but permanent, and is due in
reality to a very rare but extensive atmosphere which surrounds
the Sun, and absorbs the light which that body radiates, propor-
tionally to its thickness, which, of course, increases towards the
limb, to an observer on the Earth.

THE ENVELOPING LAYERS OF THE SUN.

The luminous surface of the Sun, or that part visible at all times,
and which forms its disk, is called the Photosphere, from the property
it is supposed to possess of generating light. The photosphere does
not extend to a great depth below the luminous surface, but forms
a comparatively thin shell, 3,000 or 4,000 miles thick, which is dis-
tinct from the interior parts, above which it seems to be kept in sus-
pense by internal forces. From the observations of some astrono-
mers it would appear that the diameter of the photosphere is subject
to slight variations, and, therefore, that the solar diameter is not a
constant quantity. From the nature of this envelope, such a result
does not seem at all impossible, but rather probable.

Immediately above the photosphere lies a comparatively thin
stratum, less than a thousand miles in thickness, called the Revers-
ing Layer. This stratum is composed of metallic vapors, which, by
absorbing the light of particular refrangibilities emanating from the
photosphere below, produces the dark Fraunhofer lines of the solar
spectrum.

Above the reversing layer, and resting immediately upon it, is a

ASTRONOMICAL DRAWINGS. 3

shallow, semi-transparent gaseous layer, which has been called the
Chromosphere^ from the fine tints which it exhibits during total
eclipses of the Sun, in contrast with the colorless white light radiated
by the photosphere below. Although visible to a certain extent on
the disk, the chromosphere is totally invisible on the limb, except
with the spectroscope, and during eclipses, on account of the nature
of its light, which is mainly monochromatic, and too feeble, com-
pared with that emitted by the photosphere, to be seen.

The chromospheric layer, which has a thickness of from 3,000 to
4,000 miles, is uneven, and is usually upheaved in certain regions,
its matter being transported to considerable elevations above its
general surface, apparently by some internal forces. The portions
of the chromosphere thus lifted up, form curious and complicated
figures, which are known under the names of Solar Protuberances, or
Solar Flames.

Above the chromosphere, and rising to an immense but unknown
height, is the solar atmosphere proper, which is only visible during
total eclipses of the Sun, and which then surrounds the dark body
of the Moon with the beautiful rays and glorious nimbus, called the
Corona.

These four envelopes : the photosphere, the reversing layer, the
chromosphere, and the corona, constitute the outer portions of our
luminary.

Below the photosphere little can be seen, although it is known,
as will appear below, that at certain depths cloud-like forms exist,
and freely float in an interior atmosphere of invisible gases. Beyond
this all is mystery, and belongs to the domain of hypothesis.

STRUCTURE OF THE PHOTOSPHERE AND CHROMOSPHERE.

The apparent uniformity of the solar surface disappears when it
is examined with a telescope of sufficient aperture and magnifying
powers. Seen under good atmospheric conditions, the greater part
of the solar surface appears mottled with an infinite number of
small, bright granules, irregularly distributed, and separated from
each other by a gray-tinted background.

These objects are known under different names. The terms
granules and granulations answer very well for the purpose, as they
do not imply anything positive as to their form and true nature.
They have also been called Luculce, Rice Grains, Willow Leaves, etc.,
by different observers.

Although having different shapes, the granulations partake more

4 THE TROUVELOT

or less of the circular or slightly elongated form. Their diameter,
which varies considerably, has been estimated at from o".5 to 3", or
from 224 to 1, 344 miles. The granulations which attain the largest
size appear, under good atmospheric conditions, to be composed of
several granules, closely united and forming an irregular mass, from
which short appendages protrude in various directions.

The number of granulations on the surface of the Sun varies con-
siderably under the action of unknown causes. Sometimes they are
small and very numerous, while at other times they are larger, less
numerous, and more widely separated. Other things being equal,
the granulations are better seen in the central regions of the Sun
than they are near the limb.

Usually the granulations are very unstable ; their relative posi-
tion, form, and size undergoing continual changes. Sometimes they
are seen to congregate or to disperse in an instant, as if acting under
the influence of attractive and repulsive forces ; assembling in
groups or files, and oftentimes forming capricious figures which are
very remarkable, but usually of short duration. In an area of great
solar disturbances, the granulations are often stretched to great dis-
tances, and form into parallel lines, either straight, wavy, or curved,
and they have then some resemblance to the flowing of viscous
liquids.

The granulations are usually terminated either by rounded or
sharply pointed summits, but they do not all rise to the same height,
as can be ascertained with the spectroscope when they are seen
sidewise on the limb. In the regions where they are most abundant,
they usually attain greater elevations, and when observed on the
limb with the spectroscope, they appear as slender acute flames.

The granulations terminated by sharply-pointed crests, although
observed in all latitudes, seem to be characteristic of certain regions.
A daily study of the chromosphere, extending over a period of ten
years, has shown me that the polar regions are rarely ever free from
these objects, which are less frequent in other parts of the Sun. In
the polar regions they are sometimes so abundant that they com-
pletely form the solar limb. These forms of granulation are com-
paratively rare in the equatorial zones, and when seen there, they
never have the permanency which they exhibit in the polar regions.
When observed in the equatorial regions, they usually appear in
small groups, in the vicinity of sun spots, or they are at least en-
closed in areas of disturbances where such spots are in process of
formation. In these regions they often attain greater elevations
than those seen in high latitudes.

ASTRONOMICAL DRA WINGS. 5

As we are certain that in the equatorial zones these slender
flames (Y. *••., granulations) are a sure sign of local disturbance, it may
be reasonably supposed that the same kind of energy producing
them nearly always prevails in the polar regions, although it is
there much weaker, and never reaches beyond certain narrow limits.

Studied with the spectroscope, the granulations are found to be
composed in the main of incandescent hydrogen gas, and of an un-
known substance provisionally called " helium." Among the most
brilliant of them are found traces of incandescent metallic vapors,
belonging to various substances found on our globe.

The chromosphere is not fixed, but varies considerably in thick-
ness in its different parts, from day to day. Its thickness is usually
greater in the polar regions, where it sometimes exceeds 6, 700 miles.
In the equatorial regions the chromosphere very rarely attains this
height, and when it does, the rising is local and occupies only a
small area. In these regions it is sometimes so shallow that its
depth is only a few seconds, and is then quite difficult to measure.
These numbers give, of course, the extreme limits of the variations
of the chromosphere ; but, nearly always, it is more shallow in the
equatorial regions ; and, as far as my observations go, the difference
in thickness between the polar and equatorial zones is greater in
years of calm than it is in years of great solar activity. But ten
years of observation are not sufficient to warrant any definite con-
clusions on this subject.

There is undoubtedly some relation between the greater thick-
ness of the chromosphere in the polar regions, and the abundance
and permanence of the sharply-pointed granulations observed in the
same regions. This becomes more evident when we know that the
appearance of similarly-pointed flames in the equatorial zones is
always accompanied with a local thickening of the chromosphere.
The thickening in the polar regions may be only apparent, and not
due to a greater accumulation of chromospheric gases there ; but
may be caused by some kind of repulsive action or polarity, which
lifts up and extends the summit of the granulations in a manner
similar to the well-known mode of electric repulsion and polarity.

As it seems very probable that the heat and light emanating
from the Sun are mainly generated at the base of the granulations, in
the filamentary elements composing the chromosphere and photo-
sphere, it would follow that, as the size and number of these objects
constantly vary, the amount of heat and light emitted by the Sun
should also vary in the same proportion.

The granulations of the solar surface are represented on Plate

6 THE TROUVELOT

I., and form the general background to the group of Sun-spots form-
ing the picture.

THE FACUL^:.

Although the solar surface is mainly covered with the luminous
granulations and the grayish background above described, it is very
rare that its appearance is so simple and uniform as already repre-
sented. For the most part, on the contrary, it is diversified by
larger, brighter, and more complicated forms, which are especially
visible towards the border of the Sun. Owing to their extraordinary
brilliancy, these objects have been called Faculcz, (torches.)

Although the faculae are very seldom seen well beyond 50 helio-
centric degrees from the limb, yet they exist, and are as numerous
in the central parts of the disk as they are towards the border ;
since they form a part of the solar surface, and participate in its
movement of rotation. Their appearance near the limb has been at-
tributed to the effect of absorption produced by the solar atmosphere
on the light from the photosphere ; but this explanation seems inad-
equate, and does not solve the problem. The well-known fact that
the solar protuberances — which are in a great measure identical with
the faculae — are much brighter at the base than they are at the
summit, perhaps gives a clue to the explanation of the phe-
nomenon; especially since we know that, in general, the summit of
the protuberances is considerably broader than their base. When
these objects are observed in the vicinity of the limb, they present
their brightest parts to the observer, since, in this position, they are
seen more or less sidewise ; and, therefore, they appear bright and
distinct. But as the faculae recede from the limb, their sides, being
seen under a constantly decreasing angle, appear more and more
foreshortened ; and, therefore, these objects grow less bright and
less distinct, until they finally become invisible, when their bases
are covered over by the broad, dusky summit generally terminating
the protuberances.

The faculae appear as bright and luminous masses or streaks on
the granular surface of the Sun, but they differ considerably in form
and size. Two types at least are distinguishable. In their simplest
form they appear either as isolated white spots, or as groups of such
spots covering large surfaces, and somewhat resembling large flakes
of snow. In their most characteristic types they appear as intensely
luminous, heavy masses, from which, in most cases, issue intricate
ramifications, sometimes extending to great distances. Generally,
the ramifications issuing from the masses of faculae have their largest

ASTRONOMICAL DRA WINGS. 1

branches directed in the main towards the eastern limb of the Sun.
Some of these branches have gigantic proportions. Occasionally
they extend over 60° and even 80° of the solar surface, and, there-
fore, attain a length of from 450,000 to 600,000 miles.

Although the faculae may be said to be seen everywhere on the
surface of the Sun, there is a vast difference in different regions,
with regard to their size, number, and brilliancy. They are largest,
most abundant, and brightest on two intermediate zones parallel to
the solar equator, and extending 35° or 40° to the north and to the
south of this line. The breadth of these zones varies considerably
with the activity of the solar forces. When they are most active,
the faculae spread on either side, but especially towards the equator,
where they sometimes nearly meet those of the other zone. In
years of little solar activity the belts formed by the faculae are very
narrow — the elements composing them being very few and small,
although they never entirely disappear.

The faculae are very unstable, and are constantly changing :
those of the small types sometimes form and vanish in a few min-
utes. When an area of disturbance of the solar surface is observed
for some time, all seems in confusion ; the movements of the granu-
lations become unusually violent ; they congregate in all sorts of
ways, and thus frequently form temporary faculae. Action of this
kind is, for the most part, peculiar to the polar regions of the Sun.

The larger faculae have undoubtedly another origin, as they seem
to be mainly formed by the ejection of incandescent gases and
metallic vapors from the interior of the photosphere. In their pro-
cess of development some of the heavy masses of faculae are swollen
up to great heights, being torn in all sorts of ways, showing large
rents and fissures through which the sight can penetrate.

Very few faculae are represented in Plate I. ; several streaks are
shown at the upper left-hand corner, some appearing as whitish
ramifications among the granulations representing the general solar
surface.

THE TROVVELOT

SUN-SPOTS AND VEILED SPOTS.
PLATE I.

BESIDES the brilliant faculae already described, much more con-
spicuous markings, though of a totally different character, are very
frequently observed on the Sun. On account of their darkish ap-
pearance, which is in strong contrast with the white envelope of
our luminary, these markings were called Macula, or Sun-spots, by
their earlier observers.

The Sun-spots are not equally distributed on the solar surface ;
but like the faculae, to which they are closely related, they occupy two
zones — one on each side of the equator. These zones are comprised
between 10° and 35° of north latitude, and 10° and 35° of south lat-
itude. Between these two zones is a belt 20° in width, where the
Sun-spots are rarely seen.

Above the latitudes 35° north and south, the Sun-spots are rare,
and it is only occasionally, and during years of great solar activity,
that they appear in these regions ; in only a few cases have spots
of considerable size been seen there. A few observers, however,
have seen spots as far as 40° and 50° from the equator ; and La
Hire even observed one in 70° of north latitude ; but these cases
are exceedingly rare. It is not uncommon, however, to see very
small spots, or groups of such spots, within 8° or 10° from the poles.

The activity of the Sun is subject to considerable fluctuation, and
accordingly the Sun-spots vary in size and number in different years.
During some years they are large, complicated, and very numer-
ous ; while in others they are 'small and scarce, and are sometimes
totally absent for weeks and months together. The fluctuations in
the frequency of Sun-spots are supposed to be periodical in their
character, although their periods do not always appear to recur at
exactly regular intervals. Sometimes the period is found to be only
nine years, while at other times it extends to twelve years. The
period generally adopted now is IITV years, nearly ; but further in-
vestigations are needed to understand the true nature of the phe-
nomenon.

ASTRONOMICAL DRAWINGS. 9

The number of Sun-spots does not symmetrically augment and
diminish, but the increase is more rapid than the diminution.

The period of increase is only about four years, while that of de-
crease is over seven years ; each period of Sun-spot maximum being
nearer the preceding period of Sun-spot minimum than it is to that
next following.

The cause of these fluctuations in the solar energy is at present
wholly unknown. Some astronomers, however, have attributed it to
the influence of the planets Venus and Jupiter, the period of revolution
of the latter planet being not much longer than the Sun-spot period;
but this supposition lacks confirmation from direct observations,
which, so far, do not seem to be in favor of the hypothesis. At the
present time the solar activity is on the increase, and the Sun-spots
will probably reach their maximum in 1883. The last minimum
occurred in 1879, when only sixteen small groups of spots were ob-
served during the whole year.

Sun-spots vary in size and appearance ; but, unless they are very
small, in which case they appear as simple black dots, they gener-
ally consist of two distinct and well-characterized parts, nearly
always present. There is first, a central part, much darker than
the other, and sharply divided from it, called the " Umbra ;" second,
a broad, irregular radiated fringe of lighter shade, completely
surrounding the first, and called the "Penumbra"

Reduced to its simplest expression, a Sun-spot is a funnel-shaped
opening through the chromosphere and the photosphere. The inner
end of the funnel, or opening, gives the form to the umbra, while
its sloping sides form the penumbra.

The umbra of Sun-spots, whose outlines approximately follow the
irregularities of the penumbral fringe, has a diameter which gener-
ally exceeds the width of the penumbral ring. Sometimes it appears
uniformly black throughout ; but it is only so by contrast, as is
proved when either Mercury or Venus passes near a spot during a
transit over the Sun's disk. The umbra then appears grayish, when
compared with the jet-black disk of the planet.

The umbra of spots is rarely so simple as just described ; but it
is frequently occupied, either partly or wholly, by grayish and rosy
forms, somewhat resem'bling loosely-entangled muscular fibres.
These forms have been called the Gray and Rosy Veils. Frequently
these veils appear as if perforated by roundish black holes, improp-
erly called Nuclei, which permit the sight to penetrate deeper into
the interior. To all appearance the gray and rosy veils are of the
same nature as the chromosphere and the faculae, and are therefore
mainly composed of hydrogen gas.

10 THE TROUVELOT

Whatever can be known about the interior of the Sun, must be
learned from the observations of these openings, which are compar-
atively small. But whatever this interior may be, we certainly know
that it is not homogeneous. Apparently, the Sun is a gigantic bub-
ble, limited by a very thin shell. Below this shell exists a large
open space filled with invisible gases, in which, through the open-
ings constituting the Sun-spots, the gray and rosy veils described
above are occasionally seen floating.

The fringe forming the penumbra of spots is much more compli-
cated than the umbra. In its simpler form, it is composed of a mul-
titude of bright, independent filaments of different forms and sizes,
partly projecting one above the other, on the sloping wall of the
penumbra, from which they seem to proceed. Seen from the Earth,
these filaments have somewhat the appearance of thatched straw,
converging towards the centre of the umbra. It is very rare, how-
ever, that the convergence of the penumbral filaments is regular, and
great confusion sometimes arises from the entanglement of these fil-
aments. Some of these elements appear straight, others are curved
or loop-shaped ; while still others, much larger and brighter than
the rest, give a final touch to this chaos of filaments, from which
results the general thatched and radiating appearance of the pe-
numbra.

The extremities of the penumbral filaments, especially of those
forming the border of the umbra, are usually club-shaped and appear
very brilliant, as if these elements had been superheated by some
forces escaping through the opening of the spots.

Besides these characteristics, the Sun-spots have others, which,
although not always present, properly belong to them. Compara-
tively few spots are so simple as the form just described. Very fre-
quently a spot is accompanied by brilliant faculae, covering part of
its umbra and penumbra, and appearing to form a part of the spot
itself.

When seen projected over Sun-spots, the faculae appear intensely
bright, and from these peculiarities they have been called Luminous
Bridges. They are, in fact, bridges, but in most cases they are at
considerable heights above the spots, kept there by invisible forces.
When such spots with luminous bridges approach the Sun's limb, it
is easy to see, by the rapid apparent displacement which they under-
go, that they are above the general level.

When the spots are closing up, the inverse effect is sometimes
observed. On several occasions, I have seen huge masses of faculae
advance slowly over the penumbra of a spot and fall into the depths

A S TRONOMICA L DRA WINGS. 11

of the umbra, resembling gigantic cataracts. I have seen narrow
branches of faculae, which, after having fallen to great depths in
the umbra, floated across it and disappeared under the photosphere
on the opposite side. I have also seen luminous bridges, resembling
cables, tightly stretched across the spots, slackening slowly, as if
loosened at one end, and gently curving into the umbra, where they
formed immense loops, large enough to receive our globe.

It is to be remarked that, in descending under- the photospheric
shell, the bright faculae and the luminous bridges gradually lose their
brilliancy. At first they appear grayish, but in descending farther
they assume more and more the pink color peculiar to the rosy veils.
The pinkish color acquired by the faculae when they reach a certain
depth under the photosphere, is precisely the color of the chromo-
sphere and of the solar protuberances, as seen during total eclipses
of the Sun — a fact which furnishes another proof that the faculae are
of the same nature as the protuberances.

I record here an observation which, at first sight, may appeal-
paradoxical ; but which seems, however, to be of considerable im-
portance, as it shows unmistakably that the solar light is mainly, if
not entirely, generated on its surface, or at least very near to it.
On May 26, 1878, I observed a large group of Sun-spots at a little
distance from the east limb of the Sun. The spot nearest to the
limb was partly covered over on its eastern and western sides by
bright and massive faculae which concealed about two-thirds of the
whole spot, only a narrow opening, running from north to south,
being left across the middle of the spot. Owing to the rotundity of
the Sun, the penumbra of this spot, although partly covered by the
faculae, could, however, be seen on its eastern side, since the sight
of the observer could there penetrate sidewise under the faculae.
Upon that part of the penumbra appeared a strong shadow, rep-
resenting perfectly the outline of the facular mass situated above
it. The phenomenon was so apparent that no error of observation
was possible, and a good drawing of it was secured. If this facula
had been as bright beneath as it was above, it is evident that no
shadow could have been produced ; hence the light of these faculae
must have been mainly generated on or very near their exterior sur-
faces. This, with the well-proved fact that the bright faculae lose
their light in falling into the interior of the Sun, seems to suggest
the idea that the bright light emitted by the faculae, and very prob-
ably all the solar light, can be generated only on its surface ; the
presence of the coronal atmosphere being perhaps necessary to pro-
duce it. Several times before this observation, I had suspected that

12 THE TROUVELOT

some faculae were casting a shadow, but as this seemed so improba-
ble, my attention was not awakened until the phenomenon became
so prominent that it could not escape notice.

With due attention, some glimpses of the phenomenon can fre-
quently be observed through the openings of some of the faculae
projecting over the penumbra of Sun-spots. It is very seldom that
the structure of the penumbra is seen through such openings, which
usually appear as dark as the umbra of the large spots, although
they do not penetrate through the photosphere like the latter. It is
only when the rents in the faculae are numerous and quite large, that
the penumbral structure is recognized through them. Since these
superficial rents in the faculae do not extend through the photo-
sphere, and appear black, it seems evident that the penumbra seen
through them cannot be as bright as it is when no faculae are pro-
jected upon it, and therefore that the faculae intercept light from the
exterior surface, which would otherwise reach the penumbra.

While the matter forming the faculae sometimes falls into the
interior of the Sun, the same kind of matter is frequently ejected in
enormous quantities, and with great force, from the interior, through
the visible and invisible openings of the photosphere, and form the
protuberances described in the following section of this manual (p.
20.) It is not only the incandescent hydrogen gas or the metallic
vapors which are thus ejected, but also cooler hydrogen gas, which
sometimes appears as dark clouds on the solar surface. On De-
cember 12, 1875, I observed such a cloud of hydrogen issuing from
the corner of a small Sun-spot. It traveled several thousand miles
on the solar surface, in a north-easterly direction, before it became
invisible.

Solar spots are formed in various ways ; but, for the most part,
the apparition of a spot is announced beforehand, by a great com-
motion of the solar surface at the place of its appearance, and by
the formation of large and bright masses of faculae, which are usu-
ally swollen into enormous bubbles by the pressure of the internal
gases. These bubbles become visible in the spectroscope while they
are traversing the solar limb, as they are then presented to us side-
wise. Under the action of the increasing pressure, the base of the
faculae is considerably stretched, and, its weakest side finally giv-
ing way, the facular mass is torn in many places from the solar sur-
face, and is perforated by holes of different sizes and forms. The
holes thus made along the border of the faculae appear as small
black spots, separated more or less by the remaining portion of the
lacerated faculae, and they enlarge more and more at the expense of

ASTRONOMICAL DRAWINGS. 13

the intervening portions, which thus become very narrow. This
perforated side of the faculse, offering less resistance, is gradually
lifted up, as would be the cover of a box, for example, while its op-
posite side remains attached to the surface. The facular matter
separating the small black holes is greatly stretched during this
action, and forms long columns and filaments. These appear as
luminous bridges upon the large and perfectly-formed spot, which is
then seen' under the lifted facular masses. The spots thus made
visible are soon freed from the facular masses, which are gradually
shifted towards the opposite side.

In such cases the spots are undoubtedly formed under the faculse
before they can be seen. This becomes evident when such spots,
not yet cleared from the faculse covering them, are observed near
the east limb ; since in this position the observer can see through
the side-openings of the faculae, and sometimes recognize the spots
under their cover.

It frequently occurs that the spots thus formed under the faculae
continue to be partly covered by the facular clouds, the forces at
work in them being apparently too feeble to shift them aside. In
such cases these spots are visible when they are in the vicinity of
the limb, where they are seen sidewise ; but when observed in the
east, in being carried forward by the solar rotation towards the
centre of the disk, they gradually diminish in size, and finally be-
come invisible. The disappearance of these spots, however, is only
apparent, being due to the fact that, as they advance towards the
centre of the disk, our lateral view of them is gradually lost, and they
are finally hidden from sight by the overhanging faculae which then
serve as a screen between the observer and the spot. This class of
spots may be called Lateral Spots, from the fact that they can
only be seen laterally, and near the Sun's limb.

Solar spots are also formed in various other ways. Some, like
those represented in Plate I., appearing without being announced
by any apparent disturbance of the surface, or by the formation of
any faculae, form and develop in a very short time. Others, appear-
ing at first as very small spots having an umbra and a penumbra,
slowly and gradually develop into very large spots. This mode of
formation, which would seem to be the most natural, is, however,
quite rare. Spots of this class have a duration and permanence not
observed in those of any other type. These spots of slow and regu-
lar development are never accompanied by faculae or luminous
bridges, nor have they any gray or rosy veils in their interior ; a
fact which may, perhaps, account for their permanent character.

14 'JHE TROUVELOT

Another class of spots, which is also rare, appear as long and
narrow crevasses showing the penumbral structure of the ordinary
spots ; but these rarely have any umbra. These long, and sometimes
exceedingly narrow fissures of the solar envelopes, with their radiated
penumbral structure, strongly suggest the idea that the photosphere
is composed of a multitude of filamentary elements having the
granulations for summits. Such a crevasse is represented on Plate
I., and unites the two spots which form the group.

The duration of Sun-spots varies greatly. Some last only for a
few hours ; while others continue for weeks and even months at a
time, but not without undergoing changes.

The modes of disappearance of Sun-spots are as various as those
of their apparition. The spots rarely close up by a gradual diminu-
tion or contraction of their umbra and penumbra. This mode of
disappearance belongs exclusively to the spots deprived of faculae
and veils. One of the most common modes of the disappearance of
a spot is its invasion by large facular masses, which slowly advance
upon its penumbra and umbra and finally cover it entirely. It is
a process precisely the reverse of that in which spots are formed by
the shifting aside of the faculae, as above described. In other types,,
the spots close up by the gradual enlargement of the luminous
bridges traversing them, which are slowly transformed into branch-
es of the photosphere, all of the characteristics of which they have
acquired. In many cases, the spots covered over by the faculae
continue to exist for some time, hidden under these masses, as is often
proved, either by the appearance of small spots on the facular mass
left at the place they occupied, or even by the reappearance of the
same spot.

Apart from the general movement of rotation of the solar sur-
face, some of the spots seem to be endowed with a proper motion of
their own, which becomes greater the nearer the spots are to the
solar equator. According to the observations of Mr. Carrington,
the period of rotation of the Sun, as deduced from the observations
of the solar spots during a period of seven years, is 25 days at the
equator ; while at 50° of heliocentric latitude it is 27 days. But the
period of rotation, as derived from the observations of spots occupy-
ing the same latitude, is far from being constant, as it varies at dif-
ferent times, with the frequency of the spots and with the solar
activity, so that at present the law of these variations is not well
known. From the character of the solar envelope, it seems very
natural that the rotation should differ in the different zones and at
different times, since this envelope is not rigid, but very movable, and
governed by forces which are themselves very variable.

ASTRONOMICAL DRAWINGS. 15

Although it is a general law that the spots near the equator
have a more rapid motion than those situated in higher latitudes,
yet, in many cases, the proper motion of the spots is more apparent
than real. For the most part, the changes of form and the rapid
displacements observed in some spots are only apparent, and due to
the fact that the large masses of faculae which are kept in suspense
above them are very unstable, and change position with the slight-
est change in the forces holding them in suspension. Since in
these cases we view the spots through the openings of the faculae
situated above them, the slightest motion of these objects produces
an apparent motion in the spots, although they have remained
motionless. Accordingly, it has been remarked that of all the
spots, those which have the greater proper motion are precisely
those which have the most faculae and luminous bridges ; while the
other spots in the same regions, but not attended by similar phe-
nomena, are comparatively steady in their movement. These last
spots are undoubtedly better adapted than any others to exhibit the
rotation of the Sun ; but it is probable that this period of rotation
will never be known with accuracy, simply because the solar surface
is unstable, and does not rotate uniformly.

The Sun-spots have a remarkable tendency to form into groups
of various sizes, but whatever may be the number of spots thus as-
sembled, the group is nearly always composed of two principal spots,
to which the others are only accessories. The tendency of the Sun-
spots to assemble in pairs is general, and is observed in all lati-
tudes, even among the minute temporary groups formed in the polar
regions. Whenever several are situated quite close together, those
belonging to the same group can be easily recognized by this char-
acter. Whatever may be the position of the axis of the two princi-
pal spots of a group when it is first formed, this axis has a decided
tendency to place itself parallel to the solar equator, no matter to
what latitude the group belongs ; and if it is disturbed from this
position, it soon returns to it when the disturbance has ceased.

It is also remarkable that the spots observed at the same time
remain in nearly the same parallel of latitude for a greater or less
period of time ; but they keep changing their position from year to
year, their latitude decreasing with the activity of the solar forces.

Among the Sun-spots, those associated with faculae form the
groups which attain the largest proportions. When such groups
acquire an apparent diameter of i' or more, they are plainly visible
to the naked eye, since for a spot to be visible to the naked eye
on the Sun, it need only subtend an angle of 50". I have some-

16 THE TROUVELOT

times seen such groups through a smoky atmosphere, when the solar
light was so much reduced that the disk could be observed directly
and without injury to the sight.

The largest spot which ever came under my observation was
seen during the period from the I3th to the iQth of November, 1870.
This spot, which was on the northern hemisphere of the Sun, was
conspicuous among the smaller spots constituting the group to
which it belonged, and followed them on the east. On November
i6th, when it attained its largest size, the diameter of its penumbra
occupied fully one-fifth of the diameter of the Sun ; its real diam-
eter being, therefore, not less than 172,000 miles, or nearly 22 times
the diameter of the Earth. As the umbra of this spot occupied a
little more than one-third of its whole diameter, seven globes like
our own, placed side by side on a straight line, could easily have
passed through this immense gap. To fill the area of this opening,
about 45 such globes would have been needed. This spot was, of
course, very easily seen with the naked eye, its diameter being
almost eight times that required for a spot to be visible without a
telescope.

Ancient historians often speak of obscurations of the Sun, and it
has been supposed by some astronomers that this phenomenon
might have been due in some cases to the apparition of large spots.
A few spots on the surface of the Sun, like that just described,
would sensibly reduce its light.

Besides the ordinary Sun-spots already described, others are at
times observed on the surface of the Sun, which show some of the
same characteristics, but never attain so large proportions. They
always appear as if seen through a fog, or veil, between the
granulations of the solar surface. On account of their vagueness and
ill-defined contours, I have proposed for these objects the term,
" Veiled Spots. " Veiled spots have a shorter duration than the
ordinary spots, the smaller types sometimes forming and vanishing
in a few minutes. Some of the larger veiled spots, however, re-
main visible for several days in succession, and show the charac-
teristics of other spots in regard to the arrangement of their parts.
The veiled spots have no umbra or penumbra, although they
are usually accompanied by faculae resembling those seen near the
ordinary spots. They are frequently seen in the polar regions, but
are there always of small size and of short duration. The veiled
spots are larger, and more apt to arrange themselves into groups,
in the regions occupied by the ordinary spots, and it is not rare to
observe such spots transform themselves into ordinary spots, and
vice versa. The veiled spots, therefore, seem to be ordinary spots

ASTRONOMICAL DRAWINGS. 17

filled up, or covered over by the granulations and semi-transparent
gases composing the chromospheric layer. That it is so, becomes
more evident, from the fact that large Sun-spots in process of dimi-
nution are sometimes gradually covered with faint and scattered
granules which descend in long, narrow filaments, and become
less and less distinguishable as they attain greater depth. This
phenomenon, associated with the fact that the luminous bridges seen
over the Sun-spots which are closing up are sometimes transformed
into branches which show the characteristic structure of the photo-
sphere, goes far to prove that the solar envelopes are mainly com-
posed of an innumerable quantity of radial filaments of varying height.

The group of Sun-spots represented in Plate I., was observed
and drawn on June i;th, 1875, at ;h. 3Om. A. M. The first traces of
this group were seen on June I5th, at noon, and consisted of three
small black dots disseminated among the granulations. At that
time, no disturbance of the surface was noticeable, and no faculae were
seen in the vicinity of these spots. On June i6th, at 8 o'clock A.M.,
the three small spots had become considerably enlarged, and, as
usual, the group consisted of two principal spots. Between these two
spots all was in motion: the granulations, stretched into long, wavy,
parallel lines, had somewhat the appearance of a liquid in rapid
motion. At I o'clock, P. M., on the same day, the group had con-
siderably enlarged; the faculae, the granulations, and the penumbral
filaments being interwoven in an indescribable manner. On the
morning of the I7th, these spots had assumed the complicated form
and development represented in the drawing ; while at the same time
two conspicuous veiled spots were seen on the left hand, at some
distance above the group.

Some luminous bridges are visible upon the left hand spot, trav-
ersing the penumbra and umbra of this spot in various directions.
The umbra of one of the spots is occupied, and partly filled with
gray and rosy veils, similar to those above described, and the granu-
lations of the solar surface form a background to the group of spots.

This group of spots was not so remarkable for its size as for its
complicated structure. The diameter of the group from east to west
was only 2% minutes of arc, or about 67,000 miles. The upper
part of the umbra of the spot situated on the right hand side of the
group was nearly 7,000 miles in diameter, or less by 1,000 miles than
the diameter of the Earth. Some of the long filaments composing
that part of the penumbra, situated on the left hand side of the same
spot, were 17,000 miles in length. One of these fiery elements would
be sufficient to encircle two-thirds of the circumference of the Earth.

18 THE TROUVELOT

SOLAR PROTUBERANCES.
PLATE II.

THE chromosphere forming the outlying envelope of the Sun, is
subject, as has been shown above, to great disturbances in cer-
tain regions, causing considerable upheavals of its surface and violent
outbursts of its gases. From these upheavals and outbursts of the
chromosphere result certain curious and very interesting forms, which
are known under the name of " Solar Protuberances" "Prominences"
or " Flame sT

These singular forms, which could, until recently, be observed
only during the short duration of the total eclipses of the Sun, can
now be seen on every clear day with the spectroscope, thanks to
Messrs. Janssen and Lockyer, to whose researches solar physics is
so much indebted.

The solar protuberances, the Sun-spots, and the faculae to which
they are closely related, are confined within the same general
regions of the Sun, although the protuberances attain higher helio-
centric latitudes.

There is certainly a very close relation between the faculae and
the solar protuberances, since when a group of the faculae traverses
the Sun's limb, protuberances are always seen at the same place. It
seems very probable that the faculae and the protuberances are in
the main identical. The faculae may be the brighter portion of the
protuberances, consisting of gases which are still undergoing a high
temperature and pressure; while the gases which have been re-
lieved from this pressure and have lost a considerable amount of
their heat, may form that part of the protuberances which is only
visible on the Sun's limb.

A daily study of the solar protuberances, continued for ten years,
has shown me that these objects are distributed on two zones which
are equidistant from the solar equator, and parallel with it. The
zone arrangement of the protuberances is more easily recognized
during the years of minimum solar activity, as in these years the

ASTRONOMICAL DRAWINGS. 19

zones are very narrow and widely separated. During these years
the belt of protuberances is situated between 40° and 45° of latitude,
north and south. In years of great solar activity the zones spread
considerably on either side of these limits, especially towards the
equator, which they nearly reach, only a narrow belt, usually free
from protuberances, remaining between them. Towards the poles
the zones do not spread so much, and there the space free from pro-
tuberances is considerably greater than it is at the equator.

During years of maximum solar activity, the protuberances, like
the Sun-spots and the faculae, are very numerous, very large, and
very complicated — sometimes occupying a great part of the whole
solar limb. As many as twenty distinct flames are sometimes ob-
served at one time. In years of minimum solar activity, on the con-
trary, the prominences are very few in number, and they are of
small size; but, as far as my observations go, they are never totally
absent.

In general, the solar flames undergo rapid changes, especially
those which are situated in the vicinity of Sun-spots, although they
occasionally remain unchanged in appearance and form for several
hours at a time. The protuberances situated in higher latitudes are
less liable to great and sudden changes, often retaining the same
form for several days. The changes observed in the protuberances
of the equatorial regions are due in part to the comparatively great
changes in their position with respect to the spectator, which are
occasioned by the rotation of the Sun. This rotation, of course, has
a greater angular velocity on the equator than in higher latitudes.
In most cases, however, the changes of the equatorial protuberances
are too great and too sudden to be thus explained. They are, in
fact, due to the greater solar activity developed in the equatorial
zones, and wherever spots are most numerous.

The solar protuberances appear under various shapes, and are
often so complicated in appearance that they defy description. Some
resemble huge clumsy masses having a few perforations on their
sides ; while others form a succession of arches supported by pillars
of different styles. Others form vertical or inclined columns, often
surmounted by cloud-like masses, or by various appendages, which
sometimes droop gracefully, resembling gigantic palm leaves. Some
resemble flames driven by the wind ; others, which are composed of
a multitude of long and narrow filaments, appear as immense fiery
bundles, from which sometimes issue long and delicate columns sur-
mounted by torch-like objects of the most fantastic pattern. Some
others resemble trees, or animal forms, in a very striking manner ;

20 THE TROUVELOT

while still others, apparently detached from the solar limb, float
above it, forming graceful streamers or clouds of various shapes.
Some of the protuberances are very massive, while others are so
thin and transparent as to form a mere veil, through which more
distant flames can easily be seen.

Notwithstanding this variety of form, two principal classes of
solar protuberances may be recognized : the cloud-like or quiescent,
and the eruptive or metallic protuberances.

The first class, which is the most common, comprises all the
cloud-like protuberances resting upon the chromosphere or floating
about it. The protuberances of this type often obtain enormous
horizontal proportions, and it is not rare to see some among them
occupying 20° and 30° of the solar limb. The height attained by
protuberances of this class does not correspond in general to their
longitudinal extent ; although some of their branches attain con-
siderable elevations. These prominences very seldom have the bril-
liancy displayed by the other type, and are sometimes so faint as to
be seen with difficulty. Although it is generally stated by observ-
ers that some of the protuberances belonging to this class are
detached from the solar surface, and kept in suspension above the
surface, like the clouds in our atmosphere, yet it seems to me very
doubtful whether protuberances are ever disconnected from the chro-
mosphere, since, in an experience of ten years, I have never been
able to satisfy myself that such a thing has occurrred. Many of
them have appeared to me at first sight to be detached from the sur-
face, but with a little patience and attention I was always able to
detect faint traces of filamentary elements connecting them with the
chromosphere. Quite often I have seen bright protuberances gradu-
ally lose their light and become invisible, while soon after they had
regained it, and were as clearly visible as before. Observations of this
kind seem to show that while the prominences are for the most part
luminous, there are also a few which are non-luminous and invisible
to the eye. These dark and invisible forms are most generally
found in the vicinity of Sun-spots in great activity. When observ-
ing such regions with the spectroscope, it is not rare to encounter
them in the form of large dark spots projecting on the solar spectrum
near the hydrogen lines. On July 28th, 1872, I observed with the
spectroscope a dark spot of this kind issuing from the vicinity of a
large Sun-spot, and extending over one-fifth of the diameter of the
Sun. This object had been independently observed in France a lit-
tle earlier by M. Chacornac with the telescope, in which it appeared
as a bluish streak.

ASTRONOMICAL DRAWINGS. 21

The second class of solar protuberances, comprising the eruptive
type, is the most interesting, inasmuch as it conveys to us a concep-
tion of the magnitude and violence of the solar forces. The protu-
berances of this class, which are always intensely bright, appear for
the most part in the immediate vicinity of Sun-spots or faculae.
These protuberances, which seem to be due to the outburst of the
chromosphere, and to the violent ejection of incandescent gases and
metallic vapors from the interior of the Sun, sometimes attain gigan-
tic proportions and enormous heights.

While the spectrum of the protuberances of the cloudy type is
simple, and usually composed of four hydrogen lines and the yellow
line D3, that of the eruptive class is very complicated, and, besides
the hydrogen lines and D3, it often exhibits the bright lines of so-
dium, magnesium, barium, titanium, and iron, and occasionally, also,
a number of other bright lines.

The phenomena of a solar outburst are grand and imposing.
Suddenly immense and acute tongues and jets of flames of a daz-
zling brilliancy rise up from the solar limb and extend in various
directions. Some of these fiery jets appear perfectly rigid, and
remain apparently motionless in the midst of the greatest disorder.
Immense straps and columns form and rise in an instant, bending
and waving in all sorts of ways and assuming innumerable shapes.
Sometimes powerful jets resembling molten metal spring up from
the Sun, describing graceful parabolas, while in their descent they
form numerous fiery drops which acquire a dazzling brilliancy when
they approach the surface.

The upward motion of the protuberances in process of formation
is sometimes very rapid. Some protuberances have been observed
to ascend in the solar atmosphere at the rate of from 120 to 497
miles a second. Great as this velocity may appear, it is nevertheless
insignificant when compared with that sometimes attained by protu-
berances moving in the line of sight instead of directly upwrards.
Movements of this kind are indicated by the displacement of the
bright or dark lines in the spectrum. A remarkable instance of this
kind occurred on the 26th of June, 1874. On that day I observed a
displacement of the hydrogen C line corresponding to a velocity of
motion of 1,600 miles per second. The mass of hydrogen gas in
motion producing such a displacement was, according to theory,
moving towards the Earth at this incredible rate, when it instantly
vanished from sight as if it had been annihilated, and was seen no
more.

Until recently the protuberances had not been observed to rise

22 THE TROUVELOT

more than 200,000 miles above the solar surface; but, on October /thr
1880, a flame, which had an elevation of 80,000 miles when I observed
it at 8h. 55m. A. M., had attained the enormous altitude of 350,000
miles when it was observed at noon by Professor C. A. Young. If
we had such a protuberance on the Earth, its summit would be at a
height sufficient not merely to reach, but to extend 100,000 miles
beyond the Moon.

Although the solar protuberances represented in Plate II. have
not the enormous proportions attained by some of these objects, yet
they are as characteristic as any of the largest ones, and afford a
good illustration of the purely eruptive type of protuberances. The
height of the largest column in the group equals 4' 43", or a little
over 126,000 miles. A large group of Sun-spots was in the vicinity
of these protuberances when they were observed and delineated.

ASTRONOMICAL DRA WINGS. 23

TOTAL ECLIPSE OF THE SUN.
PLATE III.

A SOLAR eclipse is due to the passage of the Moon directly be-
tween the observer and the Sun. Such an eclipse can only occur at
New Moon, since it is only at that time that our satellite passes be-
tween us and the Sun. The Moon's orbit does not lie precisely in the
same plane as the orbit of the Earth, but is inclined about five de-
grees to it, otherwise an eclipse of the Sun would occur at every
New Moon, and an eclipse of the Moon at every Full Moon.

Since the Moon's orbit is inclined to that of the Earth, it must
necessarily intersect this orbit at two opposite points. These points
are called the nodes of the Moon's orbit. When our satellite passes
through either of the nodes when the Moon is new, it appears inter-
posed to some extent between the Sun and the Earth, and so pro-
duces a solar eclipse ; while if it passes a node when the Moon is full,
it is more or less obscured by the Earth's shadow, which then pro-
duces an eclipse of the Moon. But, on the other hand, when the
New Moon and the Full Moon do not coincide with the passage of
our satellite through the nodes of its orbit, no eclipse can occur,
since the Moon is not then on a line with the Sun and the Earth,
but above or below that line.

Owing to the ellipticity of the Moon's orbit, the distance of our
satellite from the Earth varies considerably during each of its revo-
lutions around us, and its apparent diameter is necessarily subject to
corresponding changes. Sometimes it is greater, sometimes it is less,
than the apparent diameter of the Sun. If it is greater at the time
of a solar eclipse, the eclipse will be total to a terrestrial observer
stationed nearly on the line of the centres of the Sun and Moon,
while it will be only partial to another observer stationed further
from this line. But the Moon's distance from the Earth may be so
great and its apparent diameter consequently so small that even
those observers nearest the central line of the eclipse see the bor-
der of the Sun all round the black disk of the Moon ; the eclipse is

24 THE TROUVELOT

then annular. Even during the progress of one and the same eclipse
the distance of the Moon from the parts of the Earth towards which
its shadow is directed may vary so much that, while the eclipse is
total to some observers, others equally near the central line, but sta-
tioned at a different place, will see it as annular.

The shadow cast by the Moon on the Earth during total eclipses,
travels along upon the surface of the Earth, in consequence of the
daily movement of rotation of our globe combined with the move-
ments of the Earth and Moon in their orbits. The track of the
Moon's shadow over the Earth's surface has a general eastward
course, so that the more westerly observers see it earlier than those
east of them. An eclipse may continue total at one place for nearly
eight minutes, but in ordinary cases the total phase is much shorter.

The nodes of the Moon's orbit do not invariably occupy the
same position, but move nearly uniformly, their position with regard
to the Sun, Earth, and Moon being at any time approximately what
it formerly was at a series of times separated by equal intervals
from each other. Each interval comprises 223 lunations, or 18
years, 1 1 days, and 7 or 8 hours. The eclipses which occur within
this interval are almost exactly repeated during the next similar
interval. This period, called the " Saros," was well known to the
ancients, who were enabled by its means to predict eclipses with
some certainty.

A total eclipse of the Sun is a most beautiful and imposing
phenomenon. At the predicted time the perfectly round disk of the
Sun becomes slightly indented at its western limb by the yet invisi-
ble Moon. This phenomenon is known as the " first contact."

The slight indentation observed gradually increases with the ad-
vance of the Moon from west to east, the irregularities of the sur-
face of our satellite being plainly visible on the border of the dark
segment advancing on the Sun's disk. With the advance of the Moon
on the Sun, the light gradually diminishes on the Earth. Every
object puts on a dull and gloomy appearance, as when night is ap-
proaching; while the bright sky, losing its light, changes its pure
azure for a livid grayish color.

Two or three minutes before totality begins, the solar crescent,
reduced to minute proportions, gives comparatively so little light
that faint traces of the Sun's atmosphere appear on the western side
behind the dark body of the Moon, whose limb then becomes visible
outside of the Sun. I observed this phenomenon at Creston during
the eclipse of 1878. From 15 to 20 seconds before totality, the nar-
row arc of the Sun's disk not yet obscured by the Moon seems to

ASTRONOMICAL DRAWINGS. 25

break and separate towards the extremities of its cusps, which, thus
divided, form independent points of light, which are called "Bailys
beads'' A moment after, the whole solar crescent breaks into nu-
merous beads of light, separated by dark intervals, and, suddenly,
they all vanish with the last ray of Sunlight, and totality has begun
with the " second contact." This phenomenon of Baily's beads is
undoubtedly caused by the irregularities of the Moon's border, which,
on reaching the solar limb, divide the thin solar crescent into as
many beads of light and dark intervals as there are peaks and ravines
seen sidewise on that part of the Moon's limb.

With the disappearance of the last ray of light, the planets and
the stars of the first and second magnitude seem to light up and be-
come visible in the sky. The darkness, which had been gradually
creeping in with the progress of the eclipse, is then at its maximum.
Although subject to great variations in different eclipses, the dark-
ness is never so great as might be expected from the complete ob-
scuration of our luminary, as the part of our atmosphere which is
still exposed to the direct rays of the Sun, reflects to us some of that
light, which thus diminishes the darkness resulting from the disap-
pearance of the Sun. Usually the darkness is sufficient to prevent
the reading of common print, and to deceive animals, causing them
to act as if night was really approaching. During totality the tem-
perature decreases, while the humidity of the atmosphere augments.

Simultaneously with the disappearance of Baily's beads, a pale,
soft, silvery light bursts forth from behind the Moon, as if the Sun,
in disappearing, had been vaporized and expanded in all directions
into soft phosphorescent rays and streamers. This pale light is
emitted by gases constituting the solar atmosphere surrounding the
bright nucleus now obscured by the dark body of our satellite. This
solar atmosphere is called Corona, from its distant resemblance to
the aureola, or glory, represented by ancient painters around the
heads of saints.

With the bursting forth of the corona, a very thin arc of bright
white light is seen along the Moon's limb, where the solar crescent
has just disappeared. This thin arc of light is the reversing layer,
which, when observed with the spectroscope at that moment, exhib-
its bright lines answering to the dark lines of the ordinary solar
spectrum. Immediately above this reversing layer, and concentric
with it, appears the pink-colored chromospheric layer, with its curi-
ously shaped flames and protuberances. During totality, the chro-
mosphere and protuberances are seen without the aid of the spec-
troscope, and appear of their natural color, which, although some-

26 THE TROUVELOT

what varying in their different parts, is, on the whole, pinkish, and
similar to that of peach-blossoms; yet it is mixed here and there
with delicate prismatic hues, among which the pink and straw
colors predominate.

The color of the corona seems to vary in every eclipse, but as its
tints are very delicate, it may depend, in a great measure, upon the
vision of the observer ; although there seems to be no doubt that
there are real variations. At Creston, in 1878, it appeared to both
Professor W. Harkness and myself of a decided pale greenish hue.

The corona appears under different forms, and has never been
observed twice alike. Its dimensions are also subject to consid-
erable variations. Sometimes it appears regular and very little
extended, its distribution around the Sun being almost uniform;
although in general it spreads a little more in the direction of the
ecliptic, or of the solar equator. At other times it appears much
larger and more complicated, and forms various wings and append-
ages, which in some cases, as in 1878, extend to immense distances;
while delicate rays radiate in straight or curved lines from the spaces
left in the polar regions between the wings. The corona has some-
times appeared as if divided by immense dark gaps, apparently free
from luminous matter, and strongly resembling the dark rifts seen
in the tails of comets. This was observed in Spain and Sicily during
the total eclipse of the Sun in 1870. Different structures, forming
wisps and streamers of great length, and interlaced in various ways,
are sometimes present in the corona, while faint but more compli-
cated forms, distantly resembling enormous solar protuberances
with bright nuclei, have also been observed.

As the Moon continues its eastward progress, it gradually covers
the chromosphere and the solar protuberances on the eastern side of
the Sun; while, at the same time, the protuberances and the chro-
mosphere on the opposite limb gradually appear from under the re-
treating Moon. Then, the thin arc of the reversing layer is visible
for an instant, and is instantly followed by the appearance of a point
of dazzling white light, succeeded immediately by the apparition of
Baily's beads on each side, and totality is over, with this third con-
tact. The corona continues to be visible on the eastern side of the
Sun for several minutes longer, and then rapidly vanishes.

The thin solar crescent increases in breadth as the Moon ad-
vances; while, at the same time, the darkness and gloom spread over
nature gradually disappear, and terrestrial objects begin to resume
their natural appearance. Finally the limb of the Moon separates
from that of the Sun at the instant of " fourth contact," and the
eclipse is over.

ASTRONOMICAL DRAWINGS. 27

The phenomena exhibited by the corona in different eclipses are
very complex, and, so far, they have not been sufficiently studied to
enable us to understand the true nature of the solar atmosphere.
From the spectral analysis of the corona, and the phenomena of
polarization, it has been learned, at least, that while the matter
composing the upper part of the solar atmosphere is chiefly com-
posed of an unknown substance, producing the green line 1474, its
lower part is mainly composed of hydrogen gas at different temper-
atures, a part of which is self-luminous, while the other part only
reflects the solar light. But the proportion of the gaseous particles
emitting light, to those simply reflecting it, is subject to con-
siderable variations in different eclipses. At present it would seem
that in years of great solar disturbances, the particles emitting light
are found in greater quantity in the corona than those reflecting
it; but further observations will be required to confirm these views.

It is very difficult to understand how the corona, which in cer-
tain eclipses extends only one diameter of the Sun, should, in other
cases, as in 1878, extend to the enormous distance of twelve times
the same diameter. Changes of such magnitude in the solar atmo-
sphere, if due to the operation of forces with which we are acquaint-
ed, cannot yet be accounted for by what is known of such forces.
Their causes are still as mysterious as those concerned in the pro-
duction of the monstrous tails displayed by some comets on their
approach to the Sun.

Plate 3, representing the total eclipse of the Sun of July 29th,
1878, was drawn from my observations made at Creston, Wyoming
Territory, for the Naval Observatory. The eclipse is represented as
seen in a refracting telescope, having an aperture of 6y$ inches, and
as "it appeared a few seconds before totality was over, and when the
chromosphere was visible on the western limb of the Sun. The two
long wings seen on the east and west side of the Sun, appeared con-
siderably larger in the sky than they are represented in the picture.

28 THE TROUVELOT

THE AURORA BOREALIS.
PLATE IV.

THE name of Polar Auroras is given to certain very remarkable
luminous meteoric phenomena which appear at intervals above the
northern or the southern horizons of both hemispheres of the Earth.
When the phenomenon is produced in our northern sky, it is called
"Aurora Borealis," or " Northern Lights;" and when it appears in
the southern sky, it is called " Aurora Australis," or southern
aurora.

Marked differences appear in the various auroras observed from
our northern latitudes. While some simply consist in a pale, faint
luminosity, hardly distinguishable from twilight, others present the
most gorgeous and remarkable effects of brightness and colors.

A great aurora is usually indicated in the evening soon after
twilight, by a peculiar grayish appearance of the northern sky just
above the horizon. The grayish vapors giving that appearance, con-
tinuing to form there, soon assume a dark and gloomy aspect, while
they gradually take the form of a segment of a circle resting on the
horizon. At the same time that this dark segment is forming, a
soft pearly light, which seems to issue from its border, spreads up in
the sky, where it gradually vanishes, being the brightest at its base.
This arc of light, gradually increasing in extent as well as in bright-
ness, reaches sometimes as far as the polar star. On some rare occa-
sions, one or two, and even three, concentric arches of bright light
form one above the other over the dark segment, where they appear
as brilliant concentric rainbows. While the aurora continues to de-
velop and spread out its immense arc, the border of the dark seg-
ment loses its regularity and appears indented at several places by
patches of light, which soon develop into long, narrow, diverging
rays and streamers of great beauty. For the most part the auroral
light is either whitish or of a pale, greenish tint; but in some cases it
exhibits the most beautiful colors, among which the red and green
predominate. In these cases the rays and streamers, which are
usually of different colors, produce the most magnificent effects by
their continual changes and transformations.

ASTRONOMICAL DRAWINGS. 29

The brightness and extent of the auroral rays are likewise sub-
ject to continual changes. An instant suffices for their development
and disappearance, which may be succeeded by the sudden appear-
ance of others elsewhere, as though the original streamers had been
swiftly transported to a new place while invisible. It frequently
happens that all the streamers seem to move sidewise, from west to
east, along the arch, continuing meanwhile to exhibit their various
changes of form and color. For a time, these appearances of motion
continue to increase, a succession of streamers alternately shooting
forth and again fading, when a sudden lull occurs, during which all mo-
tion seems to have ceased. The stillness then prevailing is soon suc-
ceeded by slight pulsations of light, which seem to originate on the
border of the dark segment, and are propagated upwards along the
streamers, which have now become more numerous and active. Slow
at first, these pulsations quicken by degrees, and after a few minutes
the whole northern sky seems to be in rapid vibration. The lively
upward and downward movement of these streamers entitles them
to the name of " merry dancers" given them in northern countries
where they are frequent.

Long waves of light, quickly succeeded by others, are propa-
gated in an instant from the horizon to the zenith; these, in their
rapid passage, cause bends and curves in the streamers, which then,
losing their original straightness, wave and undulate in graceful
folds, resembling those of a pennant in a gentle breeze. Although
the coruscations add to the grandeur of the spectacle, they tend to
destroy the diverging streamers, which, being disconnected from the
dark segment, or torn in various ways, are, as it were, bodily car-
ried up towards the zenith.

In this new phase the aurora is transformed into a glorious
crown of light, called the " Corona." From this corona diverge
in all directions long streamers of different colors and forms, grace-
fully undulating in numerous folds, like so many banners of light.
Some of the largest of these streamers appear like fringes composed
of short transverse rays of different intensity and colors, producing
the most fantastic effects, when traversed by the pulsations and
coruscations which generally run across these rays during the great
auroral displays.

The aurora has now attained its full development and beauty. It
may continue in this form for half an hour, but usually the celestial
fires begin to fade at the end of fifteen or twenty minutes, reviving
from time to time, but gradually dying out. The northern sky
usually appears covered by gray and luminous streaks and patches

30 THE TROUVELOT

after a great aurora, these being occasionally rekindled, but more
often they gradually 'disappear, and the sky resumes its usual ap-
pearance.

The number of auroras which develop a corona near the zenith
is comparatively small in our latitudes ; but many of them, although
not exhibited on so grand a scale, are nevertheless very interesting.
On some very rare occasions the auroral display has been confined
almost exclusively to the dark segment, which appeared then as if
pierced along its border by many square openings, like windows,
through which appeared the bright auroral light.

Among the many auroras which I have had occasion to observe,
none are more interesting, excepting the type first described, than
those which form an immense arch of light spanning the heavens
from East to West. This form of aurora, which is quite rare, I last
observed on September I2th, 1881. All the northern sky was cov-
ered with light vapors, when a small auroral patch appeared in the
East at about 20° above the horizon. This patch of light, gradually
increasing westward, soon reached the zenith, and continued its
onward progress until it arrived at about 20° above the western
horizon, where it stopped. The aurora then appeared as a narrow,
wavy band of light, crossed by numerous parallel rays of different
intensity and color. These rays seemed to have a rapid motion
from West to East along the delicately-fringed streamer, which, on
the whole, moved southward, while its extremities remained undis-
turbed. Aside from the apparent displacement of the fringes, a
singular vibrating motion was observed in the auroral band, which
was traversed by pulsations and long waves of light. The phenom-
ena lasted for about twenty minutes, after which the arch was
broken in many places, and it slowly vanished.

The aurora usually appears in the early part of the evening, and
attains its full development between ten and eleven o'clock. Al-
though the auroral light may have apparently ceased, yet the phe-
nomenon is not at an end, as very often a solitary ray is visible from
time to time ; and even towards morning these rays sometimes be-
come quite numerous. On some occasions the phenomenon even
continues through the following day, and is manifested by the radial
direction of the cirrus-clouds in the heights of our atmosphere. In
1872 I, myself, observed an aurora which apparently continued for
two or three consecutive days and nights. In August, 1859, the
northern lights remained visible in the United States for a whole
week.

The height attained by these meteors is considerable, and it is

ASTRONOMICAL DRA WINGS. 31

now admitted that they are produced in the rarefied air of the upper
regions of our atmosphere. From the researches of Professor Elias
Loomis on the great auroras observed in August and September,
1859, it was ascertained that the inferior part of the auroral rays had
an altitude of 46 miles, while that of their summits was 428 miles.
These rays had, therefore, a length of 382 miles. From the obser-
vation of thirty auroral displays, it has been found that the mean
height attained by the summit of these streamers above the Earth's
surface was 450 miles.

But if the auroral streamers are generally manifested at great
heights in our atmosphere, it would appear from the observations of
persons living in the regions where the auroras are most frequent, as
also from those who have been stationed in high northern and
southern latitudes, that the phenomenon sometimes descends very
low. Both Sabine and Parry saw the auroral rays projected on a
distant mountain ; Ross saw them almost at sea-level projected on
the polar ice ; while Wrangel, Franklin, and others observed similar
phenomena. Dr. Hjaltalin, who has lived in latitude 64° 46' north,
and has made a particular study of the aurora, on one occasion saw
the aurora much below the summit of a hill 1,600 feet high, which
was not very far off.

The same aurora is sometimes observed on the same night at
places very far distant from one another. The great aurora borealis
of August 28th, 1859, for instance, was seen over a space occupying
150° in longitude — from California to the Ural Mountains in Russia.
It even appears now very probable that the phenomenon is universal
, on our globe, and that the northern lights observed in our hemi-
sphere are simultaneous with the aurora australis of the southern
hemisphere. The aurora of September 2d, 1859, was observed all
through North and South America, the Sandwich Islands, Austra-
lia, and Africa; the streamers and pulsations of light of the north
pole responding to the rays and coruscations of the south pole. Of
thirty-four auroras observed at Hobart Town, in Tasmania, twenty-
nine corresponded with aurora borealis observed in our hemisphere.

The auroral phenomena, although sometimes visible within the
tropics, are, however, quite rare in these regions. For the most
part they are confined within certain zones situated in high latitudes
north and south. The zone where they are most frequent in our
hemisphere forms an ellipse, which has the north pole at one of its
foci ; while the other is situated somewhere in North America, in
the vicinity of the magnetic pole. The central line of the zone upon
which the auroras seem to be most frequent passes from the north-

32 THE TROUVELOT

ern coast of Alaska through Hudson's Bay and Labrador to Iceland,,
and then follows the northern coast of Europe and Asia. The num-
ber of auroras diminishes as the observer recedes from this zone,
and it is only in exceptional cases that they are seen near the
equator. Near the pole the phenomenon is less frequent than it is-
in the region described. In North America we occupy a favorable
position for the observation of auroras, as we are nearer the magnetic
poles than are the Europeans and Asiatics, and we consequently
have a greater number of auroras in corresponding latitudes.

The position of the dark auroral segment varies with the place
occupied by the observer, and its centre always corresponds with
the magnetic meridian. In our Eastern States the auroral segment
appears a little to the west of the north point ; but as the observer
proceeds westward it gradually approaches this point, and is due
north when seen from the vicinity of Lake Winnipeg. At Point
Barrow, in the extreme northwest of the United States, the aurora
is observed in the east. In Melville Islands, Parry saw it in the
south ; while in Greenland it is directly in the west.

It is stated that auroras are more numerous about the equinoxes
than they are at any other seasons ; and also, when the earth is in
perigee, than when it is in apogee. An examination which I have
made of a catalogue by Professor Loomis, comprising 4,137 auroras
observed in the temperate zone of our hemisphere from 1776 to 1873,
sustains this statement. During this period, one hundred more
auroras were recorded during each of the months comprising the
equinoxes, than during any other months of the year ; while eighty
more auroras were observed when the earth was in perigee, than
when it was in apogee. But to establish the truth of this assertion
on a solid basis, more observations in both hemispheres will be re-
quired.

The aurora is not simply a terrestrial phenomenon, but is asso-
ciated in some mysterious way with the conditions of the Sun's-
surface. It is a well-known fact that terrestrial magnetism is
influenced directly by the Sun, which creates the diurnal oscillations
of the magnetic needle. Between sunrise and two o'clock, the north
pole of the needle moves towards the west in our northern hemi-
sphere, and in the afternoon and evening it moves the other
way. These daily oscillations of the needle are not uniform in
extent ; they have a period of regular increase and decrease.
At a given place the daily oscillations of the magnetic needle
increase and decrease with regularity during a period which is equal
to 10^ years. As this period closely coincides with the Sun-

ASTRONOMICAL DRAWINGS. 83

spot period, the connection between the variation of the needle and
these solar disturbances has been recognized.

Auroral phenomena generally accompany the extraordinary
perturbations in the oscillations of the magnetic needle, which are
commonly called " magnetic storms," and the greater the auroral
displays, the greater are the magnetic perturbations. Not only is
the needle subject to unusual displacements during an aurora, but
its movements seem to be simultaneous ^with the pulsations and
waving motions of the delicate auroral streamers in the sky. When
the aurora sends forth a coruscation, or a streamer in the sky, the
magnetic needle responds to it by a vibration. The inference that
the auroral phenomena are connected with terrestrial magnetism is
further supported by the fact that the centre of the corona is always
situated exactly in the direction of that point in the heavens to
which the dipping needle is directed.

It has been found that the aurora is a periodical phenomenon,
and that its period corresponds very closely with those of the
magnetic needle and Sun-spots. The years which have the most
Sun-spots and magnetic disturbances have also the most auroras.
There is an almost perfect similarity between the courses of the
three sets of phenomena, from which it is concluded that the aurora
is connected in some mysterious way with the action of the Sun, as
well as with the magnetic condition of the earth.

A very curious observation, which has been supposed to have
some connection with this subject, was made on Sept. 1st, 1859,
by Mr. Carrington and Mr. Hodgson, in England. While these
observers, who were situated many miles from one another, were
both engaged at the same time in observing the same Sun-spot, they
suddenly saw two luminous spots of dazzling brilliancy bursting into
sight from the edge of the Sun-spot. These objects moved eastward
for about five minutes, after which they disappeared, having then
traveled nearly 34,000 miles. Simultaneously with these appear-
ances, a magnetic disturbance was registered at Kew by the self-
registering magnetic instruments. The very night that followed
these observations, great magnetic perturbations, accompanied by
brilliant auroral displays, were observed in Europe. A connection
between the terrestrial magnetism and the auroral phenomena is
further proved by the fact that, before the appearance of an aurora,
the magnetic intensity of our globe considerably increases, but
diminishes as soon as the first flashes show themselves.

The auroral phenomena are also connected in some way with
electricity, and generate serious disturbances in the electric currents

34 THE TROUVELOT

traversing our telegraphic lines, which are thus often rendered
useless for the transmission of messages during great auroral'
displays. It sometimes happens, however, during such displays, that
the telegraphic lines can be operated for a long distance, without the
assistance of a battery ; the aurora, or at least its cause, furnishing
the necessary electric current for the working of the line. During
auroras, the telephonic lines are also greatly affected, and all kinds
of noises and crepitations are heard in the instruments.

Two observations of mine, which may have a bearing on the sub-
ject, present some interest, as they seem to indicate the action of
the aurora on some of the clouds of our atmosphere. On January
6th, 1872, after I had been observing a brilliant aurora for over
one hour, an isolated black cumulus cloud appeared at a little dis-
tance from the western extremity of the dark auroral segment.
This cloud, probably driven by the wind, rapidly advanced east-
ward, and was soon followed by a succession of similar clouds, all
starting from the same point. All these black clouds apparently
followed the same path, which was not a straight line, but paral-
lel to and concentric with the border of the dark auroral seg-
ment. When the first cloud arrived in the vicinity of the mag-
netic meridian passing through the middle of the auroral arc, it
very rapidly dissolved, and on reaching this meridian became
invisible. The same phenomenon was observed with the succession
of black clouds following, each rapidly dissolving as it approached
the magnetic meridian. This phenomenon of black clouds vanishing
like phantoms in crossing the magnetic meridian, was observed
for nearly an hour. On June i/th, 1879, I observed a similar
phenomenon during a fine auroral display. About midway between
the horizon and the polar star, but a little to the west of the
magnetic meridian, there was a large black cumulo-stratus cloud
which very slowly advanced eastward. As it progressed in that
direction, its eastern extremity was dissolved in traversing the
magnetic meridian; while, at the same time, several short and quite
bright auroral rays issued from its western extremity, which in its
turn dissolved rapidly, as if burned or melted away in the production
of the auroral flame.

It seems to be a well observed fact, that during auroras, a strong
sulphurous odor prevails in high northern latitudes. According to
Dr. Hjaltalin, during these phenomena, "the ozone of the atmosphere
increases considerably, and men and animals exposed out of doors
emit a sulphurous odor when entering a heated room." The
Esquimaux and other inhabitants of the northern regions assert

ASTRONOMICAL DRAWINGS. 35

that great auroras are sometimes accompanied by crepitations and
crackling noises of various sorts. Although these assertions have
been denied by several travelers who have visited the regions of these
phenomena, they are confirmed by many competent observers. Dr,
Hjaltalin, who has heard these noises about six times in a hundred
observations, says that they are especially audible when the weather
is clear and calm ; but that when the atmosphere is agitated they
are not heard. He compares them to the peculiar sound produced
by a silk cloth when torn asunder, or to the crepitations of the
electric machine when its motion is accelerated. " When the auroral
light is much agitated and the streamers show great movements,
it is then that these noises are heard at different places in the
atmosphere."

The spectrum of the auroral light, although it varies with al-
most every aurora, always shows a bright green line on a faint
continuous spectrum. In addition to this green line I have fre-
quently observed four broad diffused bands of greater refrangibility
in the spectra of some auroras. In two cases, when the auroras
appeared red towards the west, the spectrum showed a bright red
line, in addition to the green line and the broad bands described.
These facts evidently show that the light of the aurora is due to
the presence of luminous vapors in our atmosphere ; and it may
reasonably be supposed that these vapors are rendered luminous by
the passage of electric discharges through them.

36 THE TROUVELOT

THE ZODIACAL LIGHT.
PLATE V.

IN our northern latitudes may be seen, on every clear winter and
spring evening, a column of faint, whitish, nebulous light, rising ob-
liquely above the western horizon. A similar phenomenon may also
be observed in the east, before day-break, on any clear summer or
autumn night. To this pale, glimmering luminosity the name of
" Zodiacal Light" has been given, from the fact that it lies in the
zodiac along the ecliptic.

In common with all the celestial bodies, the zodiacal light parti-
cipates in the diurnal motion of the sky, and rises and sets with the
constellations in which it appears. Aside from this apparent motion,
it is endowed with a motion of its own, accomplished from west to
east, in a period of a year. In its motion among the stars, the
zodiacal light always keeps pace with the Sun, and appears as if
forming two faint luminous wings, resting on opposite sides of this
body. In reality it extends on each side of the Sun, its axis lying
very nearly in the plane of the ecliptic.

In our latitudes the phenomena can be observed most advan-
tageously towards the equinoxes, in March and September, when
twilight is of short duration. As we proceed southward it becomes
more prominent, and gradually increases in size and brightness. It
is within the tropical regions that the zodiacal light acquires all its
splendor : there it is visible all the year round, and always appears
very nearly perpendicular to the horizon, while at the same time its
proportions and brilliancy are greatly increased.

The zodiacal light appears under the form of a spear-head, or of
a narrow cone of light whose base apparently rests on the horizon,
while its summit rises among the zodiacal constellations. In gen-
eral appearance it somewhat resembles the tail of a large comet
whose head is below the horizon. The most favorable time to ob-
serve this phenomenon in the evening, is immediately after the last
trace of twilight has disappeared ; and in the morning, one or two

ASTRONOMICAL DRAWINGS. 37

hours before twilight appears. When observed with attention, it is
seen that the light of the zodiacal cone is not uniform, but gradually
increases in brightness inwardly, especially towards its base, where
it sometimes surpasses in brilliancy the brightest parts of the Milky-
Way. In general, its outlines are vague and very difficult to make
out, so gradually do they blend with the sky. On some favorable
occasions, the luminous cone appears to be composed of several dis-
tinct concentric conical layers, having different degrees of bright-
ness, the inner cone being the most brilliant of all. There is a re-
markable distinction between the evening and morning zodiacal
light. In our climate, the morning light is pale, and never so bright
nor so extended as the evening light.

In general, the zodiacal light is whitish and colorless, but in
some cases it acquires a warm yellowish or reddish tint. These
changes of color may be accidental and due to atmospheric condi-
tions, and not to actual change in the color of the object. Although
the zodiacal light is quite bright, and produces the impression of
having considerable depth, yet its transparency is great, since all
the stars, except the faint ones, can be seen through its substance.
The zodiacal light is subject to considerable variations in bright-
ness, and also varies in extent, the apex of its cone varying in dis-
tance from the Sun's place, from 40 to 90 degrees. These variations
cannot be attributed to atmospheric causes alone, some of them
being due to real changes in the zodiacal light itself, whose light
and dimensions increase or decrease under the action of causes at
present unknown. From the discussion of a series of observations
on the zodiacal light made at Paris and Geneva, it appears certain
that its light varies from year to year, and sometimes even from day
to day, independently of atmospheric causes. Some of my own
observations agree with these results, and one of them, at least,
seems to indicate changes even more rapid. On December i8th,
1875, I observed the zodiacal light in a clear sky free from any
vapors, at six o'clock in the evening. At that time, the point of its
cone was a little to the north of the ecliptic, at a distance of about
90 degrees from the Sun's place. Ten minutes later, its summit had
sunk down 35 degrees, the cone then being reduced to nearly one-
half of its original dimensions. Ten minutes later, it had risen 25
degrees, and was then 80 degrees from the Sun's place, where it
remained all the evening. On March 22d, 1878, the sky was very
clear and the zodiacal light was bright when I observed it, at eight
o'clock. At that moment the apex of the cone of light was a little
to the south of the Pleiades, but this cone presented an unusual

SS8 THE TROUVELOT

appearance never noticed by me before, its northern border appearing
much brighter and sharper than usual, while at the same time its
axis of greatest brightness appeared to be much nearer to this
northern border than it was to the southern. After a few minutes
of observation it became evident that the northern border was
extending itself, as stars which were at some distance from it
became gradually involved in its light. At the same time that
this border spread northward, it seemed to diffuse itself, and after a
time the cone presented its usual appearance, having its southern
border brighter and better defined than the other. It would have
been impossible to attribute this sudden change to an atmospheric
cause, since only one of the borders of the cone participated in it,
and since some very faint stars near this northern border were not
affected in the least while the phenomenon occurred. Besides these
observations, Cassini, Mairan, Humboldt, and many other competent
observers have seen pulsations, coruscations and flickerings in the
light of the cone, which they thought could not be attributed to
atmospheric causes. It has also been observed that at certain
periods the zodiacal light has shone with unusual intensity for
months together.

When this phenomenon is observed from the tropical regions, it
is found that its axis of symmetry always corresponds with its axis
of greatest brightness, and that both lie in the plane of the ecliptic,
which divides its cone into two equal parts. But when the zodiacal
light is observed in our latitude, the axis of symmetry does not
correspond with the axis of greatest brightness, and both axes are
a little to the north of this plane, the axis of symmetry being the
farther removed. Furthermore, as already stated, the southern bor-
der of the cone always appears better defined and brighter than the
corresponding northern margin. It is very probable, if not abso-
lutely certain, that these phenomena are exactly reversed when
the zodiacal light is observed from corresponding latitudes in the
southern hemisphere, and that there, its axes, both of symmetry and
of greatest brightness, appear south of the ecliptic, while the northern
margin is the brightest. This seems to be established by the valu-
able observations of Rev. George Jones, made on board the U. S.
steam frigate Mississippi, in California, Japan, and the Southern
Ocean. "When I was north of the ecliptic," says this observer,
" the greatest part of the light of the cone appeared to the north of
this line ; when I was to the south of the ecliptic, it appeared to be
south of it ; while when my position was on the ecliptic, or in its
vicinity, the zodiacal cone was equally divided by this line."

ASTRONOMICAL DRA WINGS. 39

Besides the zodiacal light observed in the East and West, some
observers have recognized an exceedingly faint, luminous, gauzy
band, about 10 or 12 degrees wide, stretching along the ecliptic from
the summit of the western to that of the eastern zodiacal cone. This
faint narrow belt has been called the Zodiacal Band. It has been
recognized by Mr. H. C. Lewis, who has made a study of this
phenomenon, that the zodiacal band has its southern margin a little
brighter and a little sharper than the northern border. This
observation is in accordance with similar phenomena observed in
the zodiacal light, and may have considerable importance.

In 1854, Brorsen recognized a faint, roundish, luminous spot in a
point of the heavens exactly opposite to the place occupied by
the Sun, which he has called " Gegenschein," or counter-glow.
This luminous spot has sometimes a small nucleus, which is a little
brighter than the rest. Night after night this very faint object shifts
its position among the constellations, keeping always at 1 80 degrees
from the Sun. The position of the counter-glow, like that of the
zodiacal light and zodiacal band, is not precisely on the plane of the
ecliptic, but a little to the north of this line. It is very probable
that near the equator the phenomenon would appear different and
there would correspond with this plane.

There seems to be some confusion among observers in regard to
the spectrum of the zodiacal light. Some have seen a bright green
line in its spectrum, corresponding to that of the aurora borealis ;
while others could only see a faint grayish continuous spectrum,
which differs, however, from that of a faint solar light, by the fact that
it presents a well-defined bright zone, gradually blending on each side
with the fainter light of the continuous spectrum. I have, myself,
frequently observed the faint continuous spectrum of the zodiacal
light, and on one occasion recognized the green line of the aurora ;
but it might have been produced by the aurora itself, as yet invisi-
ble to the eye, and not by the zodiacal light, since, later in the same
evening, there was a brilliant auroral display. If it were demon-
strated that this green line exists in the spectrum of the zodiacal
light, the fact would have importance, as tending to show that the
aurora and the zodiacal light have a common origin.

Rev. Geo. Jones describes a very curious phenomenon which he
observed several times a little before the moon rose above the hori-
zon. The phenomenon consisted in a short, oblique, luminous cone
rising from the Moon's place in the direction of the ecliptic. This
phenomenon he has called the Moon Zodiacal Light. In 1874, I had
an opportunity to observe a similar phenomenon when the Moon

40 THE TROUVELOT

was quite high in the sky. By taking the precaution to screen the
Moon's disk by the interposition of some buildings between it and
my eye, I saw two long and narrow cones of light parallel to the
ecliptic issuing from opposite sides of our satellite. The phe-
nomenon could not possibly be attributed to vapors in our atmo-
sphere, since the sky was very clear at the moment of the observa-
tion. Later on, these appendages disappeared with the formation
of vapors near the Moon, but they reappeared an hour later,
when the sky had cleared off, and continued visible for twenty min-
utes longer, and then disappeared in a clear sky.

Although the zodiacal light has been studied for over two cen-
turies, no wholly satisfactory explanation of the phenomenon has
yet been given. Now, as in Cassini's time, it is generally consid-
ered by astronomers to be due to a kind of lens-shaped ring sur-
rounding the Sun, and extending a little beyond the Earth's orbit.
This ring is supposed to lie in the plane of the ecliptic, and to be com-
posed of a multitude of independent meteoric particles circulating in
closed parallel orbits around the Sun. But many difficulties lie in
the way of this theory. It seems as incompetent to explain the
slow and rapid changes in the light of this object as it is to explain
the contractions and extensions of its cone. It fails, moreover, to
explain the flickering motions, the coruscations observed in its
light, or the displacement of its cone and of its axes of brightness
and symmetry by a mere change in the position of the observer.
Rev. Geo. Jones, unable to explain by this theory the phenomena
which came under his observation, has proposed another, which sup-
poses the zodiacal light to be produced by a luminous ring surround-
ing the Earth, this ring not extending as far as the orbit of the
Moon. But this theory also fails in many important points, so that
at present no satisfactory explanation of the phenomenon can be
given.

As the phenomenon is connected in some way with the Sun, and
as we have many reasons to believe this body to be always more or
less electrified, it might be supposed that the Sun, acting by induc-
tion on our globe, develops feeble electric currents in the rarefied
gases of the superior regions of our atmosphere, and there forms a
kind of luminous ridge moving with the Sun in a direction contrary
to the diurnal motion, and so producing the zodiacal light. On this
hypothesis, the counter-glow would be the result of a smaller cone
of light generated by the solar induction on the opposite point of
the Earth.

Plate 5, which sufficiently explains itself, represents the zodiacal

ASTRONOMICAL DRAWINGS. 41

light as it appeared in the West on the evening of February 2Oth,
1876. All the stars are placed in their proper position, and their
relative brightness is approximately shown by corresponding varia-
tions in size — the usual and almost the only available means of repre-
sentation. Of course, it must be remembered that a star does not,
in fact, show any disk even in the largest telescopes, where it ap-
pears as a mere point of light, having more or less brilliancy. The
cone of light rises obliquely along the ecliptic, and the point forming
its summit is found in the vicinity of the well-known group of stars,
called the Pleiades, in the constellation of Taurus, or the Bull.

42 THE TROUVELOT

THE MOON.

PLATE VI.

IN its endless journey through space, our globe is not solitary,
like some of the planets, but is attended by the Moon, our nearest
celestial neighbor. Although the Moon does not attain to the
dignity of a planet, and remains a secondary body in the solar
system, yet, owing to its proximity to our globe, and to the great
influence it exerts upon it by its powerful attraction, it is to us one
of the most important celestial bodies.

While the Moon accompanies the Earth around the Sun, it also
revolves around the Earth at a mean distance of 238,800 miles. For
a celestial distance this is only a trifling one ; the Earth in advanc-
ing on its orbit travels over such a distance in less than four hours.
A cannon ball would reach our satellite in nine days ; and a tele-
graphic dispatch would be transmitted there in iJ/2 seconds of time,
if a wire could be stretched between us and the Moon.

Owing to the ellipticity of the Moon's orbit, its distance from
the Earth varies considerably, our satellite being sometimes 38,000
miles nearer to us than it is at other times. These changes in the
distance of the Moon occasion corresponding changes from 29' to 33'
in its apparent diameter. The real diameter of the Moon is 2,160
miles, or a little over one-quarter the diameter of our globe ; our
satellite being 49 times smaller than the Earth.

The mean density of the materials composing the Moon is only
T% that of the materials composing the Earth, and the force of gravi-
tation at the surface of our satellite is six times less than it is at the
surface of our globe. If a person weighing 150 Ibs. on our Earth
could be transported to the Moon, his weight there would be only
25 Ibs.

The Moon revolves around the Earth in about 27*^ days, with a
mean velocity of one mile per second, the revolution constituting its
sidereal period. If the Earth were motionless, the lunar month would
be equal to the sidereal period ; but owing to its motion in space,

ASTRONOMICAL DRA WINGS. 43

the Sun appears to move with the Moon, though more slowly, so that
after having accomplished one complete revolution, our satellite has
yet to advance 2^ days before reaching the same apparent position
in regard to the Earth and the Sun that it had at first. The interval
of time comprised between two successive New Moons, which is a
little over 29^ days, constitutes the synodical period of the Moon,
or the lunar month.

The Moon is not a self-luminous body, but, like the Earth and the
planets, it reflects the light which it receives from the Sun, and so ap-
pears luminous. That such is the case is sufficiently demonstrated by
the phases exhibited by our satellite in the course of the lunar month.
Every one is familiar with these phases, which are a consequence of
the motion of the Moon around the Earth. When our satellite
is situated between us and the Sun, it is New Moon; since we can-
not see its illuminated side, which is then turned away from us
towards the Sun. When, on the contrary, it reaches that point of
its orbit which, in regard to us, is opposite to the Sun's place, it is
Full Moon ; since from the Earth we can only see the fully illumin-
ated side of our satellite. Again, when the Moon arrives at either
of the two opposite points of its orbit, the direction of which from
the Earth is at right angles with that of the Sun, it is either the
First or the Last Quarter ; since in these positions we can only see
one-half of its illuminated disk.

The curve described by the Moon around the Earth lies ap-
proximately in a plane, this plane being inclined about 5° to the
ecliptic. Since our satellite, in its motion around us and the Sun,
closely follows the ecliptic, which is inclined 23^° to the equator, it
results that when this plane is respectively high or low in the sky,
the moon is also high or low when crossing the meridian of the ob-
server. In winter that part of the ecliptic occupied by the Sun is
below the equator, and, consequently, the New Moons occurring in
that season are low in the sky, since at New Moon our satellite
must be on the same side of the ecliptic with the Sun. But the Full
Moons in the same season are necessarily high in the sky, since a
Full Moon can only occur when our satellite is on the opposite side
of the ecliptic from the Sun, in which position it is, of course, as
many degrees above the equator as the Sun is below. The Full
Moon which happens nearest to the autumnal equinox is commonly
called the Harvest Moon, from the fact that, after full, its delays in
rising on successive evenings are very brief and therefore favorable
for the harvest work in the evening. The same phenomenon occurs
in every other lunar month, but not sufficiently near the time of

44 THE TROUVELOT

Full Moon to be noticeable. When, in spring, a day or two after
New Moon, our satellite begins to show its thin crescent, its position
on the ecliptic is north as well as east of that occupied by the Sun ;
hence, its horns are nearly upright in direction, and give it a crude
resemblance to a tipping bowl, from which many people who are
unaware of its cause, and that this happens every year, draw con-
clusions as to the amount of rain to be expected.

One of the most remarkable features of the Moon's motions is
that our satellite rotates on its axis in exactly the same period of
time occupied by its revolution around the Earth, from which it
results that the Moon always presents to us the same face. To
explain this peculiarity, astronomers have supposed that the figure
of our satellite is not perfectly spherical, but elongated, so that the
attraction of the Earth, acting more powerfully upon its nearest
portions, always keeps them turned toward us, as if the Moon
were united to our globe by a string. It is not exactly true, how-
ever, that the Moon always presents its same side to us, although its
period of rotation exactly equals that of its revolution ; since in
consequence of the inclination of its axis of rotation to its orbit,
combined with the irregularities of its orbital motion about us,
apparent oscillations in latitude and in longitude, called librations,
are created, from which it results that nearly -fa of the Moon's sur-
face is visible from the Earth at one time or another.

The Moon is a familiar object, and every one is aware that our
satellite, especially when it is fully illuminated, presents a variety of
bright and dark markings, which, from their distant resemblance to-
a human face, are popularly known as " the man in the moon." A
day or two after New Moon, when the thin crescent of our satellite
is visible above the western horizon after sunset, the dark portion
of its disk is plainly visible, and appears of a pale, ashy gray color,
although not directly illuminated by the Sun. This phenomenon is
due to the Earth-shine, or to that portion of solar light which the
illuminated surface of our globe reflects to the dark side of the Moon,
exactly in the same manner that the Moon-shine, on our Earth, is-
due to the solar light reflected to our globe by the illuminated Moon.

Seen with a telescope of moderate power, or even with a good
opera-glass, the Moon presents a peculiar mottled appearance, and
has a strong resemblance to a globe made of plaster of Paris, on the
surface of which numerous roundish, saucer-shaped cavities of vari-
ous sizes are scattered at random. This mottled structure is better
seen along the boundary line called the terminator, which divides
the illuminated from the dark side of the Moon. The line of the

ASTRONOMICAL DRAWINGS. 45

terminator always appears jagged, and it is very easy to recognize
that this irregularity is due to the uneven and rugged structure of
the surface of our satellite.

A glance at the Moon through a larger telescope shows that the
bright spots recognized with the naked eye belong to very uneven
and mountainous regions of our satellite, while the dark ones belong
to comparatively smooth, low surfaces, comparable to those form-
ing the great steppes and plains of the Earth. When examined
with sufficient magnifying power, the white, rugged districts of the
Moon appear covered over by numerous elevated craggy plateaus,
mountain-chains, and deep ravines ; by steep cliffs and ridges ; by
peaks of great height and cavities of great depth. This rugged form-
ation, which is undoubtedly of volcanic origin, gives our satellite a
desolate and barren appearance. The rugged tract occupies more
than one-half of the visible surface of the Moon, forming several dis-
tinct masses, the principal of which occupy the south and south-
western part of the disk. That this formation is elevated above the
general level is proved by the fact that the mountains, peaks, and
other objects which compose it, all cast a shadow opposite to the
Sun ; and further, that the length of these shadows diminishes with
the elevation of the Sun above the lunar horizon.

Since Galileo's time the surface of the Moon has been studied by
a host of astronomers, and accurate maps of its topographical con-
figuration have been made, and names given to all features of any
prominence. It may even be said that in its general features, the
visible surface of our satellite is now better known to us than is the
surface of our own Earth.

One of the most striking and common features of the mountain-
ous districts of the Moon, is the circular, ring-like disposition of
their elevated parts, which form numerous crater-like objects of dif-
ferent sizes and depths. Many thousands of crater-like objects are
visible on the Moon through a good telescope, and, considering how
numerous the small ones are, there is, perhaps, no great exaggera-
tion in fixing their number at 50,000, as has been done by some
astronomers. These volcanic regions of the Moon cannot be com-
pared to anything we know, and far surpass in extent those of our
globe. The number and size of the craters of our most important
volcanic regions in Europe, in Asia, in North and South America,
in Java, in Sumatra, and Borneo, are insignificant when com-
pared with those of the Moon. The largest known craters on
the Earth give only a faint idea of the magnitude of some of
the lunar craters. The great crater Haleakala, in the Sandwich

46 THE TROUVELOT

Islands, probably the largest of the terrestrial volcanoes, has a cir-
cumference of thirty miles, or a diameter of a little less than ten
miles. Some of the great lunar craters, called walled plains, such
as Hipparchus, Ptolemaeus, etc., have a diameter more than ten
times larger than that of Haleakala, that of the first being 1 15 miles
and that of the last 100 miles. These are, of course, among the
largest of the craters of the Moon, although there are on our satel-
lite a great number of craters above ten miles in diameter.

The crater-forms of the Moon have evidently appeared at different
periods of time, since small craters are frequently found on the walls
of larger ones ; and, indeed, still smaller craters are not rarely seen
on the walls of these last. The walls of the lunar craters are usu-
ally quite elevated above the surrounding surface, some of them
attaining considerable elevations, especially at some points, which
form peaks of great height. Newton, the loftiest of all, rises at one
point to the height of 23,000 feet, while many others range from
ten to twenty thousand feet in height. Several craters have their
floor above the general surface — Plato, for instance. Wargentin
has its floor nearly on a level with the summit of its walls,
showing that at some period of its history liquid lavas, ejected
from within, have filled it to the brim and then solidified. The
floors of some of the craters are smooth and flat, but in general
they are occupied by peaks and abrupt mountainous masses, which
usually form the centre. Many of their outside walls are partly or
wholly covered by numerous ravines and gullies, winding down
their steep declivities, branching out and sometimes extending to
great distances from their base. It would seem that these great
volcanic mouths have at some time poured out torrents of lavas,
which, in their descent, carved their passage by the deep gullies
now visible. Sometimes, also, the crater slopes are strewn with
debris, giving them a peculiar volcanic appearance.

Notwithstanding their many points of similarity with the volca-
noes of the Earth, the lunar craters differ from them in many par-
ticulars, showing that volcanic forces acting on different globes may
produce widely different results. For example, the floors of terres-
trial craters are usually situated at considerable elevations above
the general surface, while those of the lunar craters are generally
much depressed, the height of their walls being only about one-
half the depth of their cavities. Again, while on the Earth the
mass of the volcanic cones far exceeds the capacity of their open-
ings, on the Moon it is not rare to see the capacity of the crater
cavities exceeding the mass of the surrounding walls. On the

ASTRONOMICAL DRAWINGS. 47

Earth, the volcanic cones and mouths are comparatively regular and
smooth, and are generally due to the accumulation of the ashes and
the debris of all kinds which are ejected from the volcanic mouths.
On the Moon, very few craters show this character, and for the
most part their walls have a very different structure, being irregu-
lar, very rugged, and composed of a succession of conce'ntric ridges,
rising at many points to great elevations, and forming peaks of
stupendous height. Again, many of the larger terrestrial craters
have their interior occupied by a central cone, or several such cones,
having a volcanic mouth on their summits ; on the Moon such cen-
tral cones are very rare. Although many of the large lunar craters
have their interior occupied by central masses which have been
often compared to the central cones of our great volcanoes, yet
these objects have a very different character and origin. For the
most part, they are mountainous masses of different forms — having
very rarely any craters on them — and seem to have resulted from
the crowding and lifting up of the crater floor by the phenomena
of subsidence, of which these craters show abundant signs. Besides,
the terrestrial craters are characterized by large and important lava
streams, while on the Moon the traces of such phenomena are quite
rare, and when they are shown, they generally differ from those of
the Earth by their numerous and complicated ramifications, and
also by the fact that many of these lava streamlets take their origin
at a considerable distance from the crater slopes, and are grooved
and depressed as if the burning liquids which are supposed to have
produced them had subsequently disappeared, by evaporation or
otherwise, leaving the furrow empty.

The dark spots of the Moon, when viewed through a tele-
scope, exhibit a totally different character, and show that they be-
long to a different formation from that of the brighter portions.
These darker tracts do not seem to have had a direct volcanic or-
igin like the latter, but rather appear to have resulted from the sol-
idification of'semi-fluid materials, which have overflowed vast areas
at different times. The surface of this system is comparatively
smooth and uniform, only some small craters and low ridges being
seen upon it. The level and dark appearance of these areas
led the ancient astronomers to the belief that they were produced
by a liquid strongly absorbing the rays of light, and were seas like
our seas. Accordingly, these dark surfaces were called Mariay or
Seas, a name which it is convenient to retain, although it is well
known to have originated in an error. The so-called seas of the
Moon are evidently large flat surfaces similar to the deserts, steppes,

48 THE TROUVELOT

pampas, and prairies of the Earth in general appearance. The great
plains of the Moon are at a lower level than that of the other form-
ation, and that which first attracts the observer's attention is the
fact that they are surrounded almost on all sides by an irregular line
of abrupt cliffs and mountain chains, showing phenomena of disloca-
tion. This character of dislocation, which is general, and is visible
everywhere upon the contours of the plains, seems to indicate that
phenomena of subsidence, either slow or rapid, have occurred on the
Moon ; while, at the same time, the sunken surfaces were overflowed
by a semi-fluid liquid, which solidified afterwards. The evidences of
subsidence and overflowing become unmistakable when we observe
that, along the borders of the gray plains, numerous craters are more
or less embedded in the gray formation, only parts of the summit of
their walls remaining visible, to attest that once large craters existed
there. The farther from the border of the plain the vestiges of these
craters are observed, the deeper they are embedded in the gray
formation. That phenomena of subsidence have occurred on a grand
scale on the Moon, is further indicated by the fact that the singular
systems of fractures called clefts and rifts generally follow closely
the outside border of the gray plains, often forming parallel lines of
dislocation and fractures. In the interior regions of the gray forma-
tion, these fractures are comparatively rare.

The gray, lava-like formation is obviously of later origin than
the mountainous system to which belong the embedded craters
above described. Its comparatively recent origin might also be in-
ferred from the smallness of its craters and its low ridges. The few
large craters observed on this formation evidently belong to the
earlier system.

The color of this system of gray plains is far from being uniform.
In general appearance it is of a bluish gray, but when observed at-
tentively, large areas appear tinted with a dusky olive-green, while
others are slightly tinged with yellow. Some patches appear brown-
ish, and even purplish. A remarkable example of the first case is
seen on the surface, which encloses within a large parallelogram the
two conspicuous craters, Aristarchus and Herodotus. This surface
evidently belongs to a different system from that of the Oceanus
Procellarum surrounding it, as, besides its color, which totally differs
from that of the gray formation, its surface shows the rugged struc-
ture of the volcanic formation.

When the Moon is full, some very curious white, luminous streaks
are seen radiating from different centres, which, for the most part, are
important craters, occupied by interior mountains. The great crater

ASTRONOMICAL DRA WINGS. 49

Tycho is the centre of the most imposing of the systems of white
streaks. Some of the diverging rays of this great centre extend to
a distance equal to one-quarter of the Moon's circumference, or about
i, 700 miles. The true nature of these luminous streaks is unknown,
but it seems certain that they have their origin in the crater from
which they diverge. They do not form any relief on the surface,
and are seen going up over the mountains and steep walls of the
crater, as well as down the ravines and on the floors of craters.

The Moon seems to be deprived of an atmosphere ; or, if it has any,
it must be so excessively rare that its density is less than T^ of the
density of the Earth's atmosphere, since delicate tests afforded by
the occultation of stars have failed to reveal its presence. Although
no atmosphere of any consequence exists on the Moon, yet phenom-
ena which I have observed seem to indicate the occasional presence
there of vapors of some sort. On several occasions, I have seen a
purplish light over some parts of the Moon, which prevented well-
known objects being as distinctly seen as they were at other times,
causing them to appear as if seen through a fog. One of the most
striking of these observations was made on January 4th, 1873, on
the crater Kant and its vicinity, which, then appeared as if seen
through luminous purplish vapors. On one occasion, the great cra-
ter Godin, which was entirely involved in the shadow of its western
wall, appeared illuminated in its interior by a faint purplish light,
which enabled me to recognize the structure of this interior. The
phenomenon could not be attributed in this case to reflection, since
the Sun, then just rising on the western wall of the crater, had not
yet grazed the eastern wall, which was invisible. It is not impossi-
ble that a very rare atmosphere composed of such vapors exists in
the lower parts of the Moon.

If the Moon has no air, and no liquids of any sort, it seems im-
possible that its surface can maintain any form of life, either vegeta-
ble or animal, analogous to those on the Earth. In fact, nothing
indicating life has been detected on the Moon — our satellite look-
ing like a barren, lifeless desert. If life is to be found there at all, it
must be of a very elementary nature. Aside from the want of air
and water to sustain it, the climatic conditions of our satellite are
very unfavorable for the development of life. The nights and
days of the Moon are each equal to nearly fifteen of our days and
nights. For fifteen consecutive terrestrial days the Sun's light is
absent from one hemisphere of the Moon ; while for the same num-
ber of days the Sun pours down on the other hemisphere its light
and heat, the effects of which are not in any way mitigated by an

50 THE TROUVELOT

atmosphere. During the long lunar nights the temperature must at
least fall to that of our polar regions, while during its long days it
must be far above that of our tropical zone. It has been calculated
that during the lunar nights the temperature descends to 23° below
zero, while during the days it rises to 468°, or 256° above the boil-
ing point.

It has been a question among astronomers whether changes are
still taking place at the surface of the Moon. Aside from the fact
that change, not constancy, is the law of nature, it does not seem
doubtful that changes occur on the Moon, especially in view of
the powerful influences of contraction and dilatation to which its
materials are submitted by its severe alternations of temperature.
From the distance at which we view our satellite, we cannot expect,
of course, to be able to see changes, unless they are produced on a
large scale. Theoretically speaking, the largest telescopes ever
constructed ought to show us the Moon as it would appear to the
naked eye from a distance of 40 miles ; but in practice it is very dif-
ferent. The difficulty is in the fact that, while we magnify the sur-
face of a telescopic image, we are unable to increase its light ; so
that, practically, in magnifying an object, we weaken its light pro-
portionally to the magnifying power employed. The light of the
Moon, especially near the terminator, where we almost always make
our observations, is not sufficiently bright to bear a very high mag-
nifying power, and only moderate ones can be applied to its study.
What we gain by enlarging an object, we more than lose by the
weakening of its light. Besides, a high magnifying power, by
increasing the disturbances generally present in our atmosphere,
renders the telescopic image unsteady and very indistinct. On the
whole, the largest telescopes now in existence do not show us our
satellite better than if we could see it with the naked eye from a
distance of 300 miles or more. At such a distance only considerable
changes would be visible.

Notwithstanding these difficulties, it is believed that changes
have been detected in Linne, Marius, Messier, and several other cra-
ters. An observation of mine seems to indicate that changes have
recently taken place in the great crater Eudoxus. On February
2Oth, 1877, between 9h. 3Om. and loh. 3Om., I observed a straight,
narrow wall crossing this crater from east to west, a little to the
south of its centre. This wall had a considerable elevation, as was
proved by the shadow it cast on its northern side. Towards its west-
ern end this wall appeared as a brilliant thread of light on the black
shadow cast by the western wall of the crater. The first time I had

ASTRONOMICAL DRAWINGS. 51

occasion to observe this crater again, after this observation, was a
year later, on February 1 7th, 1878 ; no traces of the wall were then
detected. Many times since I have tried to find this narrow wall
again, when the Moon presented the same phase and the same
illumination, but always with negative results. It seems probable
that this structure has crumbled down, yet it is very singular that
so prominent a feature should not have been noticed before.

The " Mare Humorum," or sea of moisture, as it is called, which
is represented on Plate VI., is one of the smaller gray lunar plains.
Its diameter, which is very nearly the same in all directions, is about
270 miles, the total area of this plain being about 50,000 square
miles. It is one of the most distinct plains of the Moon, and is
easily seen with the naked eye on the left-hand side of the disk.
The floor of the plain is, like that of tjie other gray plains, traversed
by several systems of very extended but low hills and ridges, while
small craters are disseminated upon its surface. The color of this
formation is of a dusky greenish gray along the border, while in the
interior it is of a lighter shade, and is of brownish olivaceous tint.
This plain, which is surrounded by high clefts and rifts, well illustrates
the phenomena of dislocation and subsidence. The double-ringed
crater Vitello, whose walls rise from 4,000 to 5,000 feet in height, is
seen in the upper left-hand corner of the gray plain. Close to
Vitello, at the east, is the large broken ring-plain Lee, and farther
east, and a little below, is a similarly broken crater called Doppel-
mayer. Both of these open craters have mountainous masses and
peaks on their floor, which is on a level with that of the Mare Hu-
morum. A little below, and to the left of these objects, is seen^a
deeply embedded oval crater, whose walls barely rise above the
level of the plain. On the right-hand side of the great plain, is a
long fault, with a system of fracture running along its border. On
this right-hand side, may be seen a part of the line of the terminator,
which separates the light from the darkness. Towards the lower right-
hand corner, is the great ring-plain Gassendi, 55 miles in diameter,
with its system of fractures and its central mountains, which rise from
3,000 to 4,000 feet above its floor. This crater slopes southward
towards the plain, showing the subsidence to which it has been
submitted. While the northern portion of the wall of this crater
rises to 10,000 feet, that on the plain is only 500 feet high, and is
even wholly demolished at one place where the floor of the crater
is in direct communication with the plain. In the lower part of the
mare, and a little to the west of the middle line, is found the crater
Agatharchides, which shows below its north wall the marks of rills

52 THE TROUVELOT

impressed by a flood of lava, which once issued from the side of
the crater. On the left-hand side of the plain, is seen the half-
demolished crater Hippalus, resembling a large bay, which has its
interior strewn with peaks and mountains. On this same side can
be seen one of the most important systems of clefts and fractures
visible on the Moon, these clefts varying in length from 150 to 200
miles.

ASTRONOMICAL DRAWINGS. 53

ECLIPSES OF THE MOON.
PLATE VII.

SINCE the Moon is not a self-luminous body, but shines by the light
which it borrows from the Sun, it follows that when the Sun's light is
prevented from reaching its surface, our satellite becomes obscured.
The Earth, like all opaque bodies exposed to sunlight, casts a sha-
dow in space, the direction of which is always opposite to the Sun's
place. The form of the Earth's shadow is that of a long, sharply-
pointed cone, which has our globe for its base. Its length, varying
with the distance of the Earth from the Sun, is, on an average, 855,-
ooo miles, or 108 times the terrestrial diameter. This conical shadow
of the Earth, divided longitudinally by the plane of the ecliptic, lies
half above and half below that plane, on which the summit of the
shadow describes a whole circumference in the course of a year. If
the Moon's orbit were not inclined to the ecliptic, our satellite would
pass at every Full Moon directly through the Earth's shadow ; but,
owing to that inclination? it usually passes above or below the
shadow. Twice, however, during each of its revolutions, it must
cross the plane of the ecliptic, the points of its orbit where this hap-
pens being called nodes. Accordingly, if it is near a node at the
time of Full Moon, it will enter the shadow of the Earth, and be-
come either partly or wholly obscured, according to the distance of
its centre from the plane of the ecliptic. The partial or total
obscuration of the Moon's disk thus produced constitutes a partial
or total eclipse of the Moon. The essential conditions for an
eclipse of the Moon are, therefore, that our satellite must not only
be full, but must also be at or very near one of its nodes.

Although inferior in importance to the eclipses of the Sun, the
eclipses of the Moon are, nevertheless, very interesting and remark-
able phenomena, which never fail to produce a deep impression on
the mind of the observer, inasmuch as they give him a clear insight
into the silent motions of the planetary bodies.

54 THE TROUVELOT

At the mean distance of the Moon from the Earth, the diameter
of the conical shadow cast in space by our globe is more than
twice as large as that of our satellite. But, besides this pure dark
shadow of the Earth, its cone is enveloped by a partial shadow
called " Penumbra," which is produced by the Sun's light being par-
tially, but not wholly, cut off by our globe.

While the Moon is passing into the penumbra, a slight reduction
of the light of that part of the disk which has entered it, is noticea-
ble. As the progress of the Moon continues, the reduction becomes
more remarkable, giving the impression that rare and invisible va-
pors are passing over our satellite. Some time after, a small dark
indentation, marking the instant of first contact, appears on the
eastern or left-hand border of the Moon, which is always the first to
encounter the Earth's shadow, since our satellite is moving from
west to east. The dark indentation slowly and gradually en-
larges with the onward progress of the Moon into the Earth's
shadow, while the luminous surface of its disk diminishes in the same
proportion. The form of the Earth's shadow on the Moon's disk
clearly indicates the rotundity of our globe by its circular outline.
Little by little the dark segment covers the Moon's disk, and its
crescent, at last reduced to a mere thread of light, disappears at the
moment of the second contact. With this the phase of totality be-
gins, our satellite being then completely involved in the Earth's
shadow.

The Moon remains so eclipsed for a period of time which varies
with its distance from the Earth, and with the point of its orbit
where it crosses the conical shadow. When it passes through the
middle of this shadow, while its distance from our globe is the least,
the total phase of an eclipse of the Moon may last nearly two hours.
The left-hand border of our satellite having gone first into the
Earth's shadow, is also the first to emerge, and, at the moment
of doing so, it receives the Sun's light, and totality ends with the
third contact. The lunar crescent gradually increases in breadth
after its exit from the shadow, and finally the Moon recovers
its fully illuminated disk as before, at the moment its western
border leaves the Earth's shadow. Soon after, it passes out of the
penumbra, and the eclipse is over. In total eclipses, the interval of
time from the first to last contact may last 5h. 3Om, but it is usually
shorter.

Soon after the beginning of an eclipse, the dark segment pro-
duced by the Earth's shadow on the Moon's disk generally appears
of a dark grayish opaque color, but with the progress of the phe-

ASTRONOMICAL DRAWINGS. 55

nomenon, this dark tint is changed into a dull reddish color, which,
gradually increasing, attains its greatest intensity when the eclipse
is total. At that moment the color of the Moon is of a dusky, red-
dish, coppery hue, and the general features of the Moon's surface
are visible as darker and lighter tints of the same color. It some-
times happens, however, that our satellite does not exhibit this
peculiar coppery tint, but appears either blackish or bluish, in
which case it is hardly distinguishable from the sky.

It is very rare for the Moon to disappear completely during to-
tality, and even when involved in the deepest part of the Earth's
shadow, our satellite usually remains visible to the naked eye, or, at
least, to the telescope. This phenomenon is to be attributed to the
fact that the portion of the solar rays which traverse the lower
strata of our atmosphere are strongly refracted, and bend inward in
such a manner that they fall on the Moon, and sufficiently illumin-
ate its surface to make it visible. The reddish color observed is
caused by the absorption of the blue rays of light by the vapors
which ordinarily saturate the lower regions of our atmosphere, leav-
ing only red rays to reach the Moon's surface. Of course, these
phenomena are liable to vary with every eclipse, and depend almost
exclusively on the meteorological conditions of our atmosphere.

In some cases the phase of totality lasts longer than it should,
according to calculation. This can be attributed to the fact that
the Earth is enveloped in a dense atmosphere, in which opaque
clouds of considerable extent are often forming at great elevations.
Such strata of clouds, in intercepting the Sun's light, would have,
of course, the effect of increasing the diameter of the Earth's shadow,
in a direction corresponding to the place they occupy, and, if the
Moon were moving in this direction, would increase the phase of
total obscuration.

The eclipses of the Moon, like those of the Sun, as shown above,
have a cycle of 18 years, II days and 7 hours, and recur after this
period of time in nearly the same order. They can, therefore, be
approximately predicted by adding i8y. lid. /h. to the c}ate of the
eclipses which have occurred during the preceding period. During
this cycle 70 eclipses will occur — 41 being eclipses of the Sun and
29 eclipses of the Moon. At no time can there ever be more than
seven eclipses in a year, and there are never less than two. When
there are only two eclipses in a year, they are both eclipses of the
Sun.

Although the number of solar eclipses occurring at some point
or other of the Earth's surface is greater than that of the eclipses of

56 THE TROUVELOT

the Moon, yet at any single terrestrial station the eclipses of the
Moon are the more frequent. While an eclipse of the Sun is only
visible on a narrow belt, which is but a very small fraction of the
hemisphere then illuminated by the Sun, an eclipse of the Moon is
visible from all the points of the Earth which have the Moon above
their horizon at the time. Furthermore, an eclipse of the Sun is not
visible at one time over the whole length of its narrow tract, but
moves gradually from one end of it to the other ; while, on the con-
trary, an eclipse of the Moon begins and ends at the very same
instant for all places from which it can be seen, but, of course,
not at the same local time, which varies with the longitude of the
place.

The partial eclipse of the Moon, represented on Plate VII., shows
quite plainly the configuration of our satellite as seen with the
naked eye during the eclipse, with its bright and dark spots, and its
radiating streaks. This eclipse was observed on October 24th, 1874.

ASTRONOMICAL DRA WINGS. 57

THE PLANETS.

AROUND the Sun circulate a number of celestial bodies, which
are called " Planets" The planets are opaque bodies, and appear
luminous because their surfaces reflect the light they receive from
the Sun.

The planets are situated at various distances from the Sun, and
revolve around this body in widely different periods of time, which
are, however, constant for each planet, so far as ascertained, and
doubtless are so in the other cases.

The ideal line traced in space by a planet in going around the
Sun, is called the orbit of the planet ; while the period of time em-
ployed by a planet to travel over its entire orbit and return to its
starting point, is called the sidereal revolution, or year of the planet.
The dimensions of the orbits of the different planets necessarily vary
with the distance of these bodies from the Sun, as does also the
length of their sidereal revolution.

The distance of a planet from the Sun does not remain con-
stant, but is subject to variations, which in certain cases are quite
large. These variations result from the fact that the planetary
orbits are not perfect circles having the Sun for centre, but curves
called "Ellipses" which have two centres, or foci, one of which is
always occupied by the Sun. This is in accordance with Kepler's
first law.

The ideal point situated midway between the two foci is called
the centre of the ellipse, or orbit ; while the imaginary straight line
which passes through both foci and the centre, with its ends at
opposite points of the ellipse, is called " the major axis " of the orbit.
It is also known as " the line of the apsides" The ideal straight line
which, in passing through the centre of the orbit, cuts the major
axis at right angles, and is prolonged on either side to opposite
points on the ellipse, is called " the minor axis " of the orbit.

When a planet reaches that extremity of the major axis of its or-
bit which is the nearest to the Sun, it is said to be in its "perihelion "
while, when it arrives at the other extremity, which is farthest from
this body, it is said to be in its " aphelion" When a planet reaches

58 THE TROUVELOT

either of the two opposite points of its orbit situated at the extrem-
ities of its minor axis, it is said to be at its mean distance from the
Sun.

The rapidity with which the planets move on their orbits varies
with their distance from the Sun ; the farther they are from this
body, the more slowly they move. The rapidity of their motion is
greatest when they are in perihelion, and least when they are in
aphelion, having its mean rate when these bodies are crossing either
of the extremities of the minor axes of their orbits.

The imaginary line which joins the Sun to a planet at any point
of its orbit, and moves with this planet around the Sun, is called
" the radius vector T According to Kepler's second law, whatever
may be the distance of a planet from the Sun, the radius vector
sweeps over equal areas of the plane of the planet's orbit in equal
times.

There is a remarkable relation between the distance of the
planets from the Sun and their period of revolution, in consequence
of which the squares of their periodic times are respectively equal to
the cubes of their mean distances from the Sun. From this third
law of Kepler, it results that the mere knowledge of the mean dis-
tance of a planet from the Sun enables one to know its period of
revolution, and vice versa.

The orbit described by the Earth around the Sun in a year, or
the apparent path of the Sun in the sky, is called " the ecliptic'' Like
that of all the planetary orbits, the plane of the ecliptic passes
through the Sun's centre. The ecliptic has a great importance in
astronomy, inasmuch as it is the fundamental plane to which the
orbits and motions of all planets are referred.

The orbits of the larger planets are not quite parallel to the
ecliptic, but more or less inclined to this plane ; although the incli-
nation is small, and does not exceed eight degrees. On account of
this inclination of the orbits, the planets, in accomplishing their rev-
olutions around the Sun, are sometimes above and sometimes below
the plane of the ecliptic. A belt extending 8° on each side of the
ecliptic, and, therefore, 16° in width, comprises within its limits
the orbits of all the principal planets. This belt is called "the
Zodiac?

Since all the planets have the Sun for a common centre, and have
their orbits inclined to the ecliptic, it follows that each of these
orbits must necessarily intersect the plane of the ecliptic at two op-
posite points situated at the extremities of a straight line passing
through the Sun's centre. The two opposite points on a planetary

ASTRONOMICAL DRA WINGS. 59

orbit where its intersections with the ecliptic occur, are called " the
Nodes" and the imaginary line joining them, which passes through
the Sun's centre, is called " the line of the nodes." The node situ-
ated at the point where a planet crosses the ecliptic from the south
to the north, is called "the ascending node" while that situated
where the planet crosses from north to south, is called " the descend-
ing node"

The planets circulating around the Sun are eight in number, but,
beside these, there is a multitude of very small planets, commonly
called " asteroids," which also revolve around our luminary. The
number of asteroids at present known surpasses two hundred, and
constantly increases by new discoveries. In their order of distance
from the Sun the principal planets are : Mercury, Venus, Earth,
Mars, Jupiter, Saturn, Uranus and Neptune. The orbits of the
asteroids are comprised between the orbits of Mars and Jupiter.

When the principal planets are considered in regard to their
differences in size, they are separated into two distinct groups of
four planets each, viz.: the small planets and the large planets. The
orbits of the small planets are wholly within the region occupied by
the orbits of the asteroids, while those of the large planets are
wholly without this region.

When the planets are considered in regard to their position with
reference to the Earth, they are called " inferior planets " and " su-
perior planets." The inferior planets comprise those whose orbits
are within the orbit of our globe ; while the superior planets are
those whose orbits lie beyond the orbit of the Earth.

Since the orbits of the inferior planets lie within the orbit of the
Earth, the angular distances of these bodies from the Sun, as seen
from the Earth, must always be included within fixed limits ; and
these planets must seem to oscillate from the east to the west,
and from the west to the east of the Sun during their sidereal
revolution. In this process of oscillation these planets some-
times pass between the Earth and the Sun, and sometimes
behind the Sun. When they pass between us and the Sun they
are said to be in "inferior conjunction," while, when they pass
behind the Sun, they are said to be in "superior conjunction."
When such a planet reaches its greatest distance, either east or
west, it is said to be at its greatest elongation east or west, as the
case may be, or in quadrature.

The superior planets, whose orbits lie beyond that of the Earth
and enclose it, present a different appearance. A superior planet
never passes between the Earth and the Sun, since its orbit lies

60 THE TROUVELOT

beyond that of our globe, and, therefore, no inferior conjunction of
such a planet can ever occur. When one of these planets passes
beyond the Sun, just opposite to the place occupied by the Earth,
the planet is said to be in " conjunction ;" while, when it is on the
same side of the Sun with our globe, it is said to be in " opposition."
While occupying this last position, the planet is most advantage-
ously situated for observation, since it is then nearer to the Earth.
The period comprised between two successive conjunctions, or two
successive oppositions of a planet, is called its "synodical period."
This period differs for every planet.

It is supposed that all the planets rotate from west to east, like
our globe ; although no direct evidence of the rotation of Mercury
Uranus, and Neptune has yet been obtained, it is probable that
these planets rotate like the others. It results from the rotation of
the planets that they have their days and nights, like our Earth,
but differing in duration for every planet.

The axes of rotation of the planets are more or less inclined to
their respective orbits, and this inclination varies but little in the
course of time. From the inclination of the axes of rotation of the
planets to their orbits, it results that these bodies have seasons like
those of the Earth ; but, of course, they differ from our seasons in
duration and intensity, according to the period of revolution and the
inclination of the axis of each separate planet.

ASTRONOMICAL DRAWINGS. 61

THE PLANET MARS.

PLATE VIII.

MARS is the fourth of the planets in order of distance from the
sun; Mercury, Venus and the Earth being respectively the first,
second and third.

Owing to the great eccentricity of its orbit, the distance of Mars
from the Sun is subject to considerable variations. When this planet
is in its aphelion, its distance from the Sun is 152,000,000 miles, but
at perihelion it is only 126,000,000 miles distant, the planet being
therefore 26,000,000 miles nearer the Sun at perihelion than at
aphelion. The mean distance of Mars from the Sun is 139,000,-
ooo miles. Light, which travels at the rate of 185,000 miles a second,
occupies 12^2 minutes in passing from the Sun to this planet.

While the distance of Mars from the Sun varies considerably, its
distance from the Earth varies still more. When Mars comes into
opposition, its distance from our globe is comparatively small, espe-
cially if the opposition occurs in August, as the two planets are then
as near together as it is possible for them to be, their distance apart
being only 33,000,000 miles. But if the opposition occurs in February,
the distance may be nearly twice as great, or 62,000,000 miles. On
the other hand, when Mars is in conjunction in August, the distance
between the two planets is the greatest possible, or no less than 245,-
000,000 miles; while, when the conjunction occurs in February, it is
only 216,000,000 miles. Hence the distance between Mars and the
Earth varies from 33 to 245 millions of miles; that is, this planet
may be 212 million miles nearer to us at its nearest oppositions than
at its most distant conjunctions.

From these varying distances of Mars from the Earth, necessarily
result great variations in the brightness and apparent size of the
planet, as seen from our globe. When nearest to us it is a very con-
spicuous object, appearing as a star of the first magnitude, and
approaching Jupiter in brightness; but when it is farthest it is much

62 THE TROUVELOT

reduced, and is hardly distinguishable from the stars of the second
and even third magnitude. In the first position, the apparent diam-
eter of Mars is 26", in the last it is reduced to 3" only.

The orbit of Mars has the very small inclination of i° 51' to the
plane of the ecliptic. The planet revolves around the Sun in a
period of 687 days, which constitutes its sidereal year, the year of
Mars being only 43 days less than two of our years.

Mars travels along its orbit with a mean velocity of 15 miles per
second, being about -fV of the velocity of our globe in its orbit. The
synodical period of Mars is 2 years and 48 days, during which the
planet passes through all its degrees of brightness.

Mars is a smaller planet than the Earth, its diameter being only
4,200 miles, and its circumference 13,200 miles. It seems well estab-
lished that it is a little flattened at its poles, but the actual amount
of this flattening is difficult to obtain. According to Prof. Young,
the polar compression is -5-^-.

The surface of this planet is a little over y2^ of the surface of our
globe, and its volume is 6^4 times less than that of the Earth. Its
mass is only about TV, while its density is about ^ that of the
Earth. The force of gravitation at its surface is nearly ^ of what
it is at the surface of our globe.

The planet Mars rotates on an axis inclined 61° 18' to the plane
of its orbit, so that its equator makes an angle of 28° 42' with the
same plane. The period of rotation of this planet, which constitutes
its sidereal day, is 24 h. 37 m. 23 s.

The year of Mars, which is composed of 669^3 of these Martial
days, equals 687 of our days, this planet rotating 669^3 times upon
Its axis during this period. But owing to the movement of Mars
around the Sun, the number of solar days in the Martial year is only
6682/3, while, owing to the same cause, the solar day of Mars is a lit-
tle longer than its sidereal day, and equals 24 h. 39 m. 35 s.

The days and nights on Mars are accordingly nearly of the
same length as our days and nights, the difference being a little
less than three-quarters of an hour. But while the days and nights
of Mars are essentially the same as ours, its seasons are almost twice
as long as those of the Earth. Their duration for the northern hemi-
sphere, expressed in Martial days, is as follows: Spring, 191; Sum-
mer, 181; Autumn, 149; Winter, 147. While the Spring and Sum-
mer of the northern hemisphere together last 372 days, the Autumn
and Winter of the same hemisphere last only 296 days, or 76 days
less. Since the summer seasons of the northern hemisphere corre-
spond to the winter seasons of the southern hemisphere, and vice

ASTRONOMICAL DRAWINGS. 63

versa, the northern hemisphere, owing to its longer summer, must
accumulate a larger quantity of heat than the last. But on Mars, as
on the Earth, there is a certain law of compensation resulting from
the eccentricity of the planet's orbit, and from the fact that the mid-
dle of the summer of the southern hemisphere of this planet, coin-
cides with its perihelion. From the greater proximity of Mars to
the Sun at that time, the southern hemisphere then receives more
heat in a given time than does the northern hemisphere in its summer
season. When everything is taken into account, however, it is found
that the southern hemisphere must have warmer summers and colder
winters than the northern hemisphere.

Seen with the naked eye, Mars appears as a fiery red star, whose
intensity of color is surpassed by no other star in the heavens. Seen
through the telescope, it retains the same red tint, which, however,
appears less intense, and gradually fades away toward the limb,
where it is replaced by a white luminous ring.

Mars is a very difficult object to observe, the atmosphere sur-
rounding it being sometimes so cloudy and foggy that the sight can
hardly penetrate through its vapors. When this planet is observed
under favorable atmospheric conditions, and with sufficient mag-
nifying power, its surface, which is of a general reddish tint, is found
to be diversified by white, gray and dark markings. The dark
markings, which are the most conspicuous, almost completely sur-
round the planet. They are of different forms and sizes, and very
irregular, as can be seen on Plate VIII., which represents one of the
hemispheres of this planet. Many of them, especially those situated
in the tropical regions of the planet, form long narrow bands, whose
direction is in the main parallel to the Martial equator.

The dusky spots differ very much, both from one another and in
their several parts, as regards intensity of shade. Some appear
almost black, while others which appear grayish, are so faint, that
they can seldom be seen. In the southern hemisphere, the darkest
part of the spots is generally found along their northern border;
especially where there are deep indentations.

Some observers have described these spots as being greenish or
bluish, but I have never been able to see the faintest trace of these
colors in them, except when they were observed close to the limb,
and involved in the greenish tinted ring which is always to be seen
there. It is probably an effect of contrast, since green and red are
complementary colors, and since this greenish tinge around the
limb covers all kinds of spots, whether white or dark. When such
dark spots, involved in the greenish tint, are carried by the rotation

64 THE TROUVELOT

towards the centre of the disk, they no longer show this greenish
color. To me, these spots have always appeared dark, and of such
tints as would result from a mixture of white and black in different
proportions ; except that on their lighter portions they show some of
the prevalent reddish tint of the Martial surface. It is to be remarked
that in moments of superior definition of the telescopic image, the
intensity of darkness of all the spots is considerably increased — some
of them appearing almost perfectly black.

The markings on the surface of Mars are now tolerably well
known — especially those of its southern hemisphere, which, owing to
the greater proximity of the planet to our globe when this hemis-
phere is inclined towards the earth, have been better studied. Those
of the northern hemisphere are not so well known, since when this
hemisphere is inclined towards us, the distance of Mars from the
Earth is 26,000,0x30 miles greater, so that the occasions for observing
them are not so favorable.

Several charts of Mars are in existence, but as the same nomen-
clature has not been employed in all of them, some confusion has
arisen in regard to the names given to the most remarkable features
of trie planet's surface. In order to give clearness to the subject, it
will be necessary here to give a brief description of the principal
markings represented on Plate VIII. In this the nomenclature will
be employed which has been adopted by the English observers in the
fine chart of Mr. Nath. Green. The large dark spot represented on
the left-hand side of the plate is called De La Rue Ocean. The dark
oval spot, isolated in the vicinity of the centre of the disk, is called
Terby Sea ; while the dark, irregular form on the right, near the
border, represents the western extremity of Maraldi Sea.

The dusky spots of Mars seem to be permanent, and to form a
part of the general surface of the planet. That several among them,
at least, are permanent, is proved by the fact that they have been
observed in the same position, and with the same general form, for
over two centuries. Yet, if we are to depend upon the drawings
made fifty years ago by Beer and Maedler, it would seem that the
permanency of some of them does not exist, since a very large spot
represented by these astronomers on their chart of Mars is not visi-
ble now. This object, which, on their map, has its middle at 270°,
should be precisely under the prominent dark oval spot called Terby
Sea, seen near the centre of the picture, and would extend down
almost as far as the northern limb. This can hardly be attributed
to an error of observation, since these observers were both careful,
and had great experience in this class of work. It is a very singu-

ASTRONOMICAL DRAWINGS. 65

lar fact that, at the very same place where Maedler represented the
spot in question, I found a conspicuous dark mark on December
1 6, 1 88 1, which was certainly not visible in 1877, during one of
the most favorable oppositions which can ever occur. The object,
which is still visible (Feb., 1882), consists of an isolated spot situ-
ated a little to the north of Terby Sea. During the memorable op-
position of 1877, I investigated thoroughly the markings of Mars, and
made over 200 drawings of its disk, 32 of which represent the Terby
Sea; but this isolated spot was not to be seen, unless it be identified
with the faint mark, represented on the plate, which occupied its
place. There cannot be the slightest doubt that a change has oc-
curred at that place. Changes in the markings have also been sus-
pected on the other hemisphere of the planet.

The well-known fact that the continents, and especially the
mountainous and denuded districts of our globe, reflect much more
light than the surfaces covered by water, has led astronomers to sup-
pose that the dark spots on Mars are produced by a liquid strongly
absorbing the rays of light, like the liquids on the surface of the
Earth. According to this theory the dark spots observed are sup-
posed to be lakes, seas, and oceans, similar to our own seas and
oceans, while the reddish and whitish surfaces separating these dark
spots, are supposed to be islands, peninsulas, and continents. This
supposition seems certainly to have a great deal of probability in its
favor, although some of the lighter markings may have a different
origin, and perhaps be due to vegetation ; but no observer has yet
seen in them any of the changes which ought to result from change
of seasons. Some of the changes in the dark spots might also be
' attributed to the flooding or drying up of marshes and low land.
The change which I have observed lately might be attributed to
such a cause, especially as my observation was made shortly after
the spring equinox of the northern hemisphere of Mars, which oc-
curred on December 8th.

Besides the dark spots just described, there are markings of a dif-
ferent character and appearance. Among the most conspicuous are
two very brilliant white oval spots, which always occupy opposite
sides of the planet. These two bright spots, which correspond very
closely with the poles of rotation of Mars, have been called " polar
spots."

On account of the inclination of the axis of rotation of this planet
to the ecliptic, it is rare that both of these spots are visible on the
disk at the same time ; and when this occurs, they are seen consid-
erably foreshortened, as they are then both on the limb of the planet.

66 THE TROUVELOT

Usually only one spot is visible, and it appears to its best advan-
tage when the region to which it belongs attains its maximum of in-
clination towards the Earth.

The polar spots change considerably in size, as they do also in
form. Sometimes they occupy nearly one-third of the. disk, as is
proved by many of my observations ; while at other times they are
so much reduced as to be totally invisible. It is to be remarked that
the reduction of these spots generally corresponds with the summer
seasons, and their enlargement with the winter seasons of the hemi-
spheres to which they respectively belong. From these well-observed
facts it would appear that a relation exists between the temperature
of the two hemispheres of Mars and the variations of the white spots
observed at its poles. A similar relation is known to exist on our
globe between the progress of the seasons and the melting away and
the accumulation of snow in the polar regions. Astronomers have
been led, accordingly, to attribute the polar spots of Mars, with all
their variations, to the alternate accumulation and melting of snows.
On this account, the polar spots of Mars are sometimes also called
" snow-spots."

Errors have certainly been made by astronomers in some of their
observations of the so-called polar snow-spots, other objects occu-
pying their place having been mistaken for them. A regular series
of observations on this planet, which I have now continued for seven
years, has revealed the fact that during the winter seasons of the
southern hemisphere of Mars, the polar spots are most of the time
invisible, being covered over by white, opaque, cloud-like forms,
strongly reflecting light. In 1877, during more than a month, I,
myself, mistook for the polar spots such a canopy of clouds, which
covered at least one-fifth of the surface of the whole disk. I only
became aware of my error when the opaque cloud, beginning to dis-
solve at the approach of the Martial summer, allowed the real polar
spot to be seen through its vapors, as through a mist at first, and
afterwards with great distinctness. In this particular case, the snow-
spot was considerably smaller than the cloudy cap which covered it,
and it is to be remarked that it was not situated at the centre of this
cloudy cap, but was east of that centre ; a fact which may account
for the so-called polar spots not being always observed on exactly
opposite sides of the disk. From my observations of 1877, 1878 and
1880, it appears that at the approach of the autumnal equinox of the
southern hemisphere of Mars, large, opaque masses, like cumulus clouds
in form, began to gather in the polar regions of that hemisphere, and
continued through autumn and winter, dissolving only at the ap-

ASTRONOMICAL DRAWINGS, 67

proach of spring. These clouds, which varied in form and extent,
were very unsteady at first, but as the winter drew nearer they
enlarged and became more permanent, covering large surfaces for
months at a time.

That the large white spots under consideration are real clouds in
the atmosphere of Mars, and are not due to a fall of snow, is proved
by the fact that these spots covered both seas and continents with
equal facility, even in the equatorial regions of the planet. Snow,
of course, could not cover the seas of Mars, unless these were all
frozen over, even in the equatorial zones ; therefore, if the dark spots
of Mars are assumed to be due to water, these large white spots can-
not well be ascribed to snow.

The real polar spots of Mars seem to be in relief on the surface
of the planet, since the southern spot often appeared slightly shaded
on the side opposite to the Sun during my observations in 1877. In
certain cases, when they are on or very near the limb, they have
been observed, both by others and by myself, to project from the
disk slightly.

The polar spots of Mars are doubtless composed of a material
which, like our snow or ice, melts under the rays of the Sun ; although
it seems difficult to admit that the Martial snow is identical with our
terrestrial snow, and that it melts at a like temperature. The south
polar spot of Mars entirely disappeared from sight in its summer sea-
son in 1877, although the planet receives less than one-half as much
heat as we receive from the Sun ; yet on our globe the arctic or ant-
arctic ices and snow are perpetual — never melting entirely. An im-
portant fact disclosed by the melting away of the southern polar
spot is, that in melting it is always surrounded with a very dark sur-
face, which takes the place of the melted portion of the spot, as
observed by myself in i877~'78. When the polar spot had entirely
disappeared, its place was occupied by a very dark spot. Now, if
the polar spot is really ice, and the dark spots are actual seas, this
polar spot must be situated in mid-ocean, since, on melting away, it
is replaced by a dark spot. If the polar spots are composed of a white
substance melting under the rays of the Sun, as seems altogether
probable, its melting point must be above that of terrestrial snow.

Many of the dark spots of Mars, and especially those whose
northern border forms an irregular belt upon the equatorial regions
of this planet, are bordered on that side by a white luminous belt,
following all their sinuosities. These white borders are variable.
Sometimes they are very prominent and intensely bright, especially
at some points, which occasionally almost equal the polar spots in

68 THE TROUVELOT

brilliancy; while at other times they are so faint, that they can
hardly be distinguished, or are even invisible; although the atmos-
phere is clear and the dark spots appear perfectly well defined.
While these white borders were invisible, I have sometimes watched
for several hours at a time to see if I could detect any traces of them
in places where they usually appear the most prominent, but gen-
erally without success. On a few occasions, however, I had the
good fortune to see some of these spots forming gradually in the
course of one or two hours, at places where nothing of them could
be seen before.

These whitish fringes forming and vanishing along the coasts of the
Martial seas have been very little studied by astronomers. From
my observations made during the last seven years, it appears very
probable that this belt and its white spots are mainly due to the
condensation of vapors around, and over high peaks, and extensive
mountain chains, forming the Martial sea-coasts, as the Andes and
Rocky Mountains form the sea-coasts of the Pacific Ocean. These
high mountains on Mars, condensing the vapors into fogs or clouds
above them, or at their sides, as often happens in our mountainous
districts, would certainly suffice to produce the phenomena observed.
Some of the highest peaks among these mountain chains may even
have their summits covered with perpetual snow, or some substance
partaking of the nature of snow. The temporary visibility and in-
visibility of the white spots seen on Mars, as well as the rapid trans-
formations they sometimes undergo, may be explained as caused by
clouds having a high reflective power and a liability to form and
disappear quickly.

The assumption that these irregular whitish bands and spots are
formed by the condensation of vapors on mountain chains, and ele-
vated table lands, is supported by my observations made in 1877
and 1879. When such white spots were traversing the terminator
at sunrise, they very often projected far into the night side, thus in-
dicating that they were at a higher level than that of the general
surface. Indentations in the terminator, corresponding to large
dark spots crossing its line, also clearly indicated the depression of
the dark spots below the general surface. The highest mountain-
ous districts thus observed on Mars, are situated between 60° and
70° of south latitude, towards the western extremity of Gill Land.
The mountain chain, which almost completely forms the surface of
this land, is so elevated at some points, that they not only change
the form of the terminator when they are seen upon it, but also
the limb of the planet, as seen by myself. They then appear so

ASTRONOMICAL DRAWINGS. 69

brilliant, that the principal summit among them has been mis-
taken by several observers for the polar spot itself, as proved by
the wrong position assigned to it on their drawings. It seems prob-
able that this high peak, which appears always white, is constantly
covered with snow, or the similar material replacing it on Mars.
This high region is situated between longitudes 180° and 190°.

The highest mountainous parts belonging to the hemisphere rep-
resented on Plate VIII., which are nearly always more or less visible
as whitish spots and bands, form a coast line along the northern
(lower) border of De La Rue Ocean. This great spot, which is not
so simple as it has been represented by observers, is in fact divided
by two narrow isthmuses, one in the north, the other in the east,
both joining, in the interior of the great ocean, a peninsula hereto-
fore known as Hall Island. Upon the south-eastern extremity of
this peninsula, a white spot, called Dawes Ice Island, was observed
in 1865, but it soon disappeared, and was after that seen only now
and then. It is very probable that this so-called Ice Island was
due to clouds forming around the summit of some high peak of this
peninsula.

On the opposite hemisphere to that represented on Plate VIII.,
the white fringes bordering the dark spots are much more con-
spicuous than they are on this side. On the eastern side of a re-
markable dark spot called Kaiser Sea, they are very bright, and
almost always present, although they vary considerably, both in
brightness and in extent. To the south of Kaiser Sea, they are very
conspicuous on the eastern border of Lockyer Land, forming an ele-
vated and deeply indented coast-line along Lambert Sea. There
the white spots never disappear entirely, being always visible on the
north side, where they turn westward along Dawes' Ocean — the
mountain chain attaining there its greatest altitude. Very frequently
Lockyer Land, which seems to be a vast plateau, appears throughout
white and brilliant, this occurring usually towards the sunrise or sun-
set of that region, probably from the condensation of vapors and the
formation of fogs, but generally this whiteness gradually disappears
with the progress of the sun above this plateau. Inside of the great
continents of Mars these temporary white spots are not so frequent,
but when visible they occupy always the same positions — a fact
which probably indicates that they occupy the culminating points of
these continents. One of these temporary white spots inside of the
continents is represented on Plate VIII., on the left-hand side, below
De La Rue Ocean, on Maedler Continent.

Although large, opaque, cumulus-shaped, cloud-like forms are seen

70 THE TROUVELOT

in the polar regions of Mars, such forms are very seldom seen in the
tropical zones, or, at least, it appears so, from the fact that my observ-
ations, continued during the last seven years, have disclosed no real
opaque cloudy forms there. Although the Martial sky is frequently
overcast by dense vapors or thick fogs in these regions, yet no
real opaque clouds were ever seen ; the most prominent among the
dusky spots being faintly visible through the vapory veil, when
they approached the centre of the disk.

Besides these phenomena, which prove that Mars is surrounded
by an atmosphere having a great deal of similarity to our own, a
further proof is afforded by the fact that the dark spots, which appear
sharply defined and black when they are seen near the centre, be-
come less and less visible as they advance towards the limb, and are
totally invisible before they reach it. Moreover, the spectroscope
also indicates the existence of an atmosphere, and even the presence
of watery vapor in it. A very curious state of the Martial atmos-
phere is revealed by my observations of 1877-78. During eight con-
secutive weeks, from December I2th to February 6th, a whole hem-
isphere of the planet — precisely that represented on Plate VIII.—
was completely covered by dense vapors, or a thick fog which barely
allowed the dark spots to be seen through it, even when they were
in the centre of the disk. The opposite hemisphere of Mars appeared
just as clear and calm as possible ; there all the spots and their mi-
nutest details could be seen, and when the planet was observed at the
proper time, the line separating the foggy from the clear side was
plainly visible.

The reddish tint observed on the continents of Mars has been
supposed by some astronomers to be the real color of the atmos-
phere of this planet. But, for many reasons, this explanation is not
acceptable. Besides the fact that the border of the planet appears
white, while it should be more red than the other part, owing to the
greater depth of atmosphere there presented to us, the polar spots,
the white bands along the sea-coasts, and the cloud-like forms ap-
pear perfectly white, not the slightest tint of red being visible on
them, as would be the case if these objects were seen through an
atmosphere tinted red. Other astronomers have supposed that the
vegetation of this planet has a reddish color ; but this is not sup-
ported by observation. It has been again supposed, with much more
probability, that the surface of Mars is composed of an ochreous mate-
rial which gives the planet its predominant ruddy color.

Until lately Mars was supposed to be without a satellite, but in
August, 1877, Professor Hall, of the Washington Observatory, made

ASTRONOMICAL DRA WINGS. 71

one of the most remarkable discoveries of the time, and found two
satellites revolving around this planet. These satellites are among
the smallest known heavenly bodies, their diameter having been esti-
mated at from 6 to 10 miles for the outer satellite, and from 10 to
40 miles for the inner one.

The most extraordinary feature of these bodies is the proximity
of the inner satellite to the planet, and the consequent rapidity of
its motion. The distance of the inner satellite from the centre of
Mars is about 6,000 miles, and from surface to surface it is less than
4,000 miles, or a little more than the distance from New York to
San Francisco. The shortest period of revolution of any satel-
lite previously known, is that of the inner satellite of Saturn, which
is a little more than 22^2 hours ; but the inner satellite of Mars ac-
complishes its revolution in /h. 38m., or in 17 hours less than the
period of rotation of the planet upon its axis. The period of revo-
lution of the outer satellite is greater, of course, and equals 3Oh. /m.

From this rapidity of motion of the inner satellite of Mars, a very
curious result follows, which at first sight may appear in contradic-
tion with the fact that this body has a direct motion, like that of all
the planets of the solar system, and moves around Mars from west
to east. While the outer satellite of this planet, in company with
all the stars and planets, rises in the east and sets in the west, the
inner satellite, on the contrary, rises in the west and sets in the
east. Since the period of rotation of Mars is greater than is the
period of revolution of this satellite, it necessarily follows that this
last body must constantly be gaining on the rotation, and, conse-
quently, that the satellite sets in the east and rises in the west,
compassing the whole heavens around Mars three times a day, pass-
ing through all its phases in 1 1 hours, each quarter of this singular
Moon lasting less than 3 hours.

It has been shown above that Mars has many points of resem-
blance to the Earth. It has an atmosphere constituted very nearly
like ours ; it has fogs, clouds, rains, snows, and winds. It has water,
or at least some liquids resembling it ; it has rivers, lakes, seas
and oceans. It has also islands, peninsulas, continents, mountains
and valleys. It has two Moons, which must create great and rapid
tides in the waters of its seas and oceans. It has its days and nights,
its warm and cold seasons, and very likely its vegetation, its prairies
and forests, like the Earth. On the other hand, its year and sea-
sons are double those of the Earth, and its distance from the Sun
is greater.

Is this planet, which is certainly constituted very nearly like our

72

THE IROUVELOT

globe, and seems so nearly fitted for the wants of the human race,
inhabited by animals and intelligent beings ?

To answer this question, either in the negative or in the affirma-
tive, would be to step out of the pure province of science, and enter
the boundless domain of speculation, since no observer has ever seen
anything indicating that animal life exists on Mars, or on any other
planet or satellite. So far as observation goes, Mars seems to be a
planet well suited to sustain animal life, and we may reason from
analogy that if animal life can exist at all outside of the Earth, Mars
must have its flora and fauna ; it must have its fishes and birds, its
mammalia and men ; although all these living beings must inevita-
bly be very different in appearance from their representatives on the
Earth, as can easily be imagined from the differences existing be-
tween the two planets. Although all this is possible, and even very
probable, yet it must be remembered that we have not the slightest
evidence that it is so ; and until we have acquired this evidence, we
may only provisionally accept this idea as a pleasing hypothesis,
which, after all, may be wrong and totally unfounded.

ASTRONOMICAL DRAWINGS. 73

THE PLANET JUPITER
PLATE IX.

JUPITER, the giant of the planetary world, is the fifth in order of
distance from the Sun, and is next to Mars, our ruddy neighbor. To
the naked eye, Jupiter appears as a very brilliant star, whose mag-
nitude, changing with the distance of this planet from the Earth,
sometimes approaches that of Venus, our bright morning and even-
ing star.

The mean distance of Jupiter from the Sun is 475,000,000 miles,
but owing to the eccentricity of its orbit, its distance varies from 452
to 498 millions of miles. The distance of this planet from the Earth
varies still more. When nearest to our globe, or in opposition, its
distance is reduced to 384,000,000 miles, and its apparent diameter
increased to 50"; while when it is farthest, or in conjunction, its dis-
tance is increased to 567,000,000 miles, and its apparent diameter
reduced to 30"; Jupiter being thus 183,000,000 miles nearer our globe
while in opposition than when it is in conjunction.

This planet revolves around the Sun in 1 1 years, 10 months and
17 days, or in only 50 days less than 12 terrestrial years. Such is the
year of this planet. The plane of its orbit is inclined i° 19' to the
ecliptic. No planet, except Uranus, has an orbit exhibiting a
smaller inclination. The planet advances in its orbit at the mean
rate of 8 miles a second; which is a little less than half the orbital
velocity of the Earth.

Jupiter is of enormous proportions. Its equatorial diameter meas-
ures 88,000 miles, and its circumference no less than 276,460 miles,
these dimensions being 1 1 times greater than those of the Earth.
This planet, notwithstanding its huge size, rotates on its axis in not
far from 9h. 55m. 363., which period constitutes its day. Owing, how-
ever, to the changeable appearance of its surface, this period cannot
be ascertained with very great exactitude. In consequence of its
rapid rotation, the planet is far from spherical, its polar diameter
being shorter than the equatorial by about T^, or 5,500 miles. Its

74 THE TROVVELOT

surface is 124 times the surface of the earth; while its volume is
1,387 times as great. If Jupiter occupied the place of our satellite
in the sky, it would appear 40 times as large as the Moon appears
to us, and would cover a surface of the heavens 1,600 times that
covered by the full Moon, and would subtend an angle of 21°.
Jupiter's mass does not correspond with its great bulk, and is only
Y^j7 of the mass of the Sun, and 310 times the mass of the earth;
its density being only % of that of our globe. The force of gravita-
tion at the surface of this planet is over 2^ times what it is on the
P^arth, so that a terrestrial object carried to the surface of Jupiter
would weigh over two and a half times as much as on our globe.

Observed with a telescope, even of moderate aperture, Jupiter,
with its four attending satellites and its dazzling brilliancy, appears
as one of the most magnificent objects in the sky. The general ap-
pearance of the disk is white; but unlike that of Mars, it is brightest
towards its central parts, and a little darker around the limb, espe-
cially on the side opposite to the Sun. Although an exterior planet,
and so far from us, Jupiter shows faint traces of phases when ob-
served near its quadratures, but this gibbosity of its disk is very
slight, and is indicated only by a kind of penumbral shadow on the
limb.

When observed with adequate power, the disk of Jupiter is found to
be highly diversified. The principal features consist of a series of al-
ternate light and dark streaks or bands, disposed most of the time
parallel with the Jovian equator. These bands differ from each
other in intensity as well as in breadth; those near the equator be-
ing usually much more prominent than those situated in higher lati-
tudes north and south.

The equatorial zone of Jupiter is occupied most of the time by a
broad, prominent belt 20° or 30° wide, limited on each side by a
very dark narrow streak. Between these two dark borders, but sel-
dom occupying the whole space between them, appears an irregular
white belt, apparently composed of dense masses of clouds strongly
reflecting the Sun's light, some of these cloudy masses being very
brilliant. The spaces left between the cloudy belt and the dark
borders, usually exhibit a delicate pink or rosy color, which pro-
duces a very harmonious effect with the varying grayish and bluish
shades of some of the belts and streaks seen on the disk. Quite
often the cloudy belt is broken up, and consists of independent
cloudy masses, separated by larger or smaller intervals, these inter-
vals disclosing the rosy background of this zone.

On each side of the equatorial belt there is usually a broad

ASTRONOMICAL DRAWINGS. 75

whitish belt, succeeded by a narrow gray band; the space left on
each hemisphere between these last bands and the limb being usu-
ally occupied by two or three alternate white and gray bands. A
uniform gray segment usually forms a sort of polar cap to Jupiter.

When observed under very favorable conditions, all the lighter
belts appear as if composed of masses of small cloudlets, resembling
the white opaque clouds seen in our atmosphere. This, as already
stated, is particularly noticeable on the equatorial belt. It is not
unusual, when Jupiter is in quadrature, to see some of the most con-
spicuous white spots casting a shadow opposite to the Sun; a fact
which sufficiently indicates that these spots are at different levels.
They probably form the summits of vast banks of clouds floating
high up in the atmosphere of Jupiter.

What we see of Jupiter is chiefly a vaporous, cloudy envelope.
If our sight penetrates anywhere deeper into the interior, it can only
be through the narrow fissures of this envelope, which appear as
gray or dark streaks or spots. That most of the visible surface of
Jupiter is simply a cloudy covering, is abundantly proved by the
proper motion of its spots, which sometimes becomes very great.

In periods of calm, very few changes are noticeable in the mark-
ings of the planet, except, perhaps, some slight modifications of form
in the cloudy, equatorial belt which, in general, is much more liable
to changes than the other belts. But the Jovian surface is not al-
ways so tranquil, great changes being observed during the terrific
storms which sometimes occur on this mighty planet, when all be-
comes disorder and confusion on its usually calm surface; and noth-
ing on the Earth can give us a conception of the velocity with
which some of its clouds and spots are animated. New belts quickly
form, while old ones disappear. The usual parallelism of the belts
no more exists. Huge, white, cumulus-like masses advance and
spread out, the rosy equatorial belt enlarges sometimes to two or
three times its usual size, and occupies two-thirds, or more, of the
disk, the rosy tint spreading out in a very short time. At times
very dark bands extending across the disk are transformed into
knots or dark spots, which encircle the planet with a belt, as it
were, of jet black beads. Sometimes, also, a secondary but nar-
rower rosy belt forms either in the northern or the southern hemi-
sphere, and remains visible for a few days or for years at a time.

On May 25, 1876, I witnessed one of the grandest commotions
which can be conceived as taking place in an atmosphere. All the
southern hemisphere of Jupiter, from equator to pole, was in rapid
motion, the belts and spots being transported entirely across the

76 THE TROUVELOT

disk, from the eastern to the western limb, in one hour's time, during-
which the equatorial belt swelled to twice its original breadth,
towards the south.

Now, when one stops for a moment to think what is signified by
that motion of the dark spots across the little telescopic disk of Jupi-
ter in an hour's time, he may arrive at some conception of the mag-
nitude of the Jovian storms, compared with those of our globe. The
circumference of Jupiter's equator, as stated above, is 276,460 miles;
half this number, or 138,230 miles, represents the length of the
equatorial line seen from the Earth. Now, after taking into account
the rotation of the planet, which somewhat diminishes the apparent
motion, we arrive at the astonishing result that the spots and mark-
ings were carried along by this Jovian storm, at the enormous rate
of 1 10,584 miles an hour, or over 30.7 miles a second. On our globe, a
hurricane or tornado, which blows at the rate of 100 miles an hour,
sweeps everything before it. What, then, must be expected from a
velocity over 1,105 times as great ? Enormous as this motion may
appear, its occurrence cannot be doubted, since it is disclosed by
direct observation.

The surface of Jupiter, it would seem, has its periods of calm and
activity like that of the Sun, although it is not yet known, as it is
for the latter, that they recur with approximate regularity.

My observations of this planet, which embrace a period of ten
years, seem to point in that direction, for they show, at least, that
Jupiter has its years of calm and its years of disturbances. The year
1876 was a year of extraordinary disturbance on Jupiter. Changes
in the markings were going on all the time, and no one form could
be recognized the next day, or even sometimes the next hour, as
shown above. The cloudy envelope of the planet was in constant
motion, the equatorial belt, especially, showing the signs of greatest
disturbance, being, for the most part, two or three times as wide as
in other years. After 1876 the calm was very great on the planet,
only a slight change now and then being noticeable, the same forms
being recognized day after day, month after month, and even year
after year. In one case the same marking has been observed for
seventeen consecutive months, and in another for twenty-eight
months. This state of quietude lasted until October, 1880, when
considerable commotion occurred on the northern hemisphere, where
large round black spots, somewhat resembling the Sun-spots, formed
in the cloudy atmosphere, and finally changed, towards the end of
December, into a narrow pink belt, which still exists.

The most curious marking ever seen on Jupiter is undoubtedly

ASTRONOMICAL DRAWINGS. 77

the great Red Spot, observed on the southern hemisphere of this
planet for the last three years. This interesting object, seen first in
July, 1878, disappeared for a time, reappeared on September 25 of the
same year, and has remained visible until now. When seen by me
in September, it was much elongated, and sharply pointed on one
side, like a spear-head, but it subsequently acquired an irregular form,
with short appendages protruding from its northern border. At
first, the changes were great and frequent, but at length it acquired
the regular oval form, which, with but slight modifications, it has
retained until now. During the month of November, 1880, I noticed
two small black specks upon this Red Spot, and they were seen again
in January of the succeeding year, by Mr. Alvan Clark, Jr. When the
spot had attained its oval shape, it appeared part of the time sur-
rounded with a white luminous ring of cloudy forms which, however,
was changing more or less all the time, being sometimes invisible.
The color of this curious spot is a brilliant rosy red, tinged with ver-
milion, and altogether different in shade from the pinkish color of
the equatorial belt. The size of the spot varies, but of late its
changes have been slight. Its longer diameter may be estimated at
8,000 miles, and the shorter at 2,200 miles. The Red Spot is repre-
sented on Plate IX. with its natural color, and as it appears at the
moments most favorable for observation. In ordinary cases its color
does not appear so brilliant, but paler.

It is difficult to account for the color of the equatorial belt and
that of the Red Spot; but it is known, at least, that the material to
which they are due cannot be situated at the level of the general
surface visible to us, and especially that of the cloudy forms of the
equatorial zone. Undoubtedly the red layer lies deeper than the
superficial envelope of the planet, although it does not seem to be
very deeply depressed.

Jupiter is attended by four satellites, which revolve around the
planet at various distances, and shine like stars of the 6th and /th
magnitude. It is said that under very favorable circumstances, and
in a very clear sky, the satellites can be seen with the naked eye,
but this requires exceptionally keen eyes, since the glare of the
planet is so strong as to overpower the comparatively faint light of
the satellites. However, I myself have sometimes seen, without the
aid of the telescope, two or three of the satellites ' as a single
object, when they were closely grouped on the same side of Jupiter.

The four moons of Jupiter are ail larger than our Moon, except
the first, which has about the same diameter. They range in size
from 2,300 to 3,400 miles in diameter, the third being the largest ;

78 THE TROUVELOT

the determination of their diameter is by no means accurate, how-
ever, as it is difficult to measure such small objects with precision.
Their mean distance from the centre of Jupiter varies from 267,000
to 1,192,000 miles, the first satellite, the nearest to the planet,
being a little farther from Jupiter then our satellite is from us. The
four satellites revolve around the planet in orbits whose planes have
a slight inclination to the equator of Jupiter, and consequently to the
ecliptic. The diameter of the largest satellite is nearly half that of
the Earth, or 3,436 miles; while its volume is five times that of our
Moon. The period of revolution of these satellites varies from id.
i8h. for the first, to i6d. i6h. for the last.

Owing to the slight inclination of the pla,ne of their orbits to that
of the planet, the three first satellites, and generally the fourth,
pass in front of the disk and also through the shadow of the planet
at every revolution, and are accordingly eclipsed. Their passages
behind Jupiter's disk are called occultations; those in front of it,
transits. The eclipses, the occultations and the transits of the moons
of Jupiter are interesting and important phenomena ; the eclipses
being sometimes observed for the rough determination of longitudes
at sea.

The satellites in transit present some curious phenomena. When
they enter the disk, they appear intensely luminous upon its gray-
ish border ; but as they advance, they seem by degrees to lose
their brightness, until they finally become undistinguishable from
the luminous surface of Jupiter. It sometimes happens, however,
that the first, the third and the fourth satellites, after ceasing to
appear as bright spots, continue to be visible as dark spots upon
the bright central portions of the planet's disk ; but in these cases
their disks appear smaller than the shadows they cast. Undoubt-
edly these satellites have extensive atmospheres, since they some-
times pass unperceived across the central parts of Jupiter, this being
probably when their atmospheres are condensed into clouds, strong-
ly reflecting light ; while when these clouds are absent, we can see
their actual surface, with traces of the dark spots upon them similar
to those on Mars.

From the variation in the brightness of these satellites, which is
said to be always observed in the same part of their orbit, William
Herschel was" led to suppose that these bodies, like our Moon,
rotate upon their axes in the same period in which they move round
the planet, so that they always present the same face to Jupiter ;
but these conclusions have been denied. From my observations
it is apparent, however, that the light reflected by them varies in

ASTRONOMICAL DRAWINGS. 79

intensity as well as in color. But this is rather to be attributed to
the presence of an atmosphere surrounding these bodies, which
when cloudy reflects more light than when clear, with correspond-
ing changes in the color of the light.

The satellites in transit are sometimes preceded or followed,
according to the position of the Sun, by a round black spot having
about the same size as the satellite itself. This black spot is the
shadow of the satellite cast on the vapory envelope of Jupiter,
similar to the shadow cast by the Moon on the Earth, during
eclipses of the Sun ; in fact, all the Jovian regions traversed by
these shadows have the Sun totally eclipsed. Sometimes it happens
that the shadow appears elliptical. This occurs either when it is
observed very near the limb, or when entering upon a round, cloud-
like spot. This effect is attributable to the perspective under which
the shadow is seen on the spherical globe or spot.

The proper motion of the satellites in the Jovian sky is much
more rapid than that of the Moon in our sky. During one Jovian
day of ten hours, the first satellite advances 84° ; the second, 42° ;
the third, 20° and the fourth, 9°. The first satellite passes from
New Moon to its first quarter in a little more than a Jovian day,
while the fourth occupies ten such days in attaining the same phase.

In density, as well as in physical constitution, Jupiter differs
widely from the interior planets, and especially from the Earth; and,
as has been shown, it is surrounded by a dense, opaque, cloudy layer,
which is almost always impenetrable to the sight, and hides from
view the nucleus, which we may conceive to exist under this va-
porous envelope. In 1876, the year of the great Jovian disturbances,
I observed frequently in the northern hemisphere of the planet a
very curious phenomenon, which seems to prove that its cloudy en-
velope is at times partially absent in some places, its vapors being
apparently either condensed, or transported to other parts of its sur-
face, and that, therefore, a considerable part of the real globe of the
planet was visible at these places. The phenomenon consisted in
the deformation of the northern limb, which had a much shorter ra-
dius on all of this hemisphere situated northward of the white belt
which adjoins the equatorial zone. The deformation of the limb
on both sides, where it passed from a longer to a shorter radius,
was abrupt, and at right angles to the limb, forming there a step-
like indentation which was very prominent. The polar segment
having a smaller radius, appeared unusually dark, and was not
striped, as usual, but uniform in tint throughout. On September
27th, the third satellite passed over this dark segment, and emerged

80 THE TROUVELOT

from the western border, a little below the place where the limb
was abruptly deformed, as above described. When the satellite had
fully emerged from this limb, it was apparent that if the portion of
the limb having a longer radius had been prolonged a little below,
and as far as the satellite, it would have enclosed it within its bor-
der, and thus retarded the time of emersion. The depth of defor-
mation of the limb was accordingly greater than the diameter of the
third satellite, and certainly more than 4,000 miles. That the phe-
nomenon was real, is proved by the fact that the egress of this sat-
ellite occurred at least four minutes sooner than the time predicted
for it in the American Ephemeris. Other observations seem to
point in the same direction, since some of the satellites which were
occulted have been seen through the limb of Jupiter by different as-
tronomers, as if this limb was sometimes semi-transparent. Another
observation of mine seems to confirm these conclusions. On April
24th, 1877, at I5h. 25m. the shadow of the first satellite was pro-
jected on the dark band forming the northern border of the equa-
torial belt, the shadow being then not far from the east limb. Close
to this shadow, and on its western side, it was preceded by a sec-
ondary shadow, which was fainter, but had the same apparent size.
This round dark spot was not the satellite itself, as I had supposed
at first, since this object was yet outside of the planet, on the east,
and entered upon it only at i6h. 4m. I watched closely this strange
phenomenon, and at i6h. 45m., when the shadow had already crossed
about ^ of the disk, it was still preceded by the secondary, or mock
shadow, as it may be called; the same relative distance having been
kept all the while between the two objects, which had therefore
traveled at the same rate. It is obvious that this dark spot could
not be one of the planet's markings, since the shadow of the first
satellite moves more quickly on the surface of Jupiter than a spot on
the same surface travels by the effect of rotation, so that in this case
the shadow would soon have passed over this marking, and left it
behind, during the time occupied by the observation. From these
observations it seems very probable that Jupiter has a nucleus, either
solid or liquid, which lies several thousand miles below the surface
of its cloudy envelope. It is also probable that the uniformly shaded
dark segment seen in 1876, was a portion of the surface of this nu-
cleus itself. When the cloudy envelope is semi-transparent at the
place situated on a line with an occulted satellite and the eye of an
observer, this satellite may accordingly remain visible for a time
through the limb, as shown by observation. The phenomenon of
the mock shadow may also be attributed to a similar cause, where

ASTRONOMICAL DRA WINGS. 81

semi-transparent vapors receive the shadow of a satellite at their
surface, while at the same time part of this shadow, passing through
the semi-transparent vapors, may be seen at the surface of the nu-
cleus, or of a layer of opaque clouds situated at some distance below
the surface.

Some astronomers are inclined to think that Jupiter is at a high
temperature, and self-luminous to a certain extent. If this planet is
self-luminous to any degree, we might expect that some light would
be thrown upon the satellites when they are crossing the shadow
cast into space by the planet; but when they cross this shadow they
are totally invisible in the best telescopes, a proof that they do not
receive much light from the non-illuminated side of Jupiter. It
would, indeed, seem probable that some of the intensely white spots
occasionally seen on the equatorial belt of the planet are self-lumin-
ous in a degree, yet not enough to render the satellites visible while
they are immersed in Jupiter's shadow. It does not seem impossible
that the planet should have the high temperature attributed to it,
when we remember the terrific storms observed in its atmosphere,
which, owing to the great distance of Jupiter from the Sun, do not
seem to be attributable to this body, but rather to some local cause
within the envelope of the planet.

Astronomy, which is a science of observation, is naturally silent
with regard to the inhabitants of Jupiter. If there are any such in-
habitants, they are confined to the domain of conjecture, under the
dense cloudy envelope of the planet. The conditions of habitability
on Jupiter must differ very widely from those of our globe. Com-
paratively little direct light from the Sun reaches the surface of the
globe of Jupiter, except that which passes through the narrow
openings forming the dark clouds. All the rest of the planet's sur-
face, being covered perpetually by opaque clouds, receives only dif-
fused light. On Jupiter there are practically no seasons, since its
axis is nearly perpendicular to its orbit. The force of gravity on the
surface of Jupiter being more than double what it is on the Earth,
living bodies would there have more than double the weight of sim-
ilar bodies on the Earth. Furthermore, Jupiter only receives o.oii
of the light and heat which we receive from the Sun ; and its
year is nearly equal to 12 of our years. If there are living beings on
Jupiter, they must, then, be entirely different from any known to us,
and they may have forms never dreamed of in our most fantastic
conceptions.

The two round black spots represented towards the central parts
of Plate IX. are the shadows of the first and second satellite ; while

82

THE TROUVELOT

the two round white spots seen on the left of the disk, are the satel-
lites themselves, as they appeared at the moment of the observa-
tion. The first satellite and its shadow are the nearest to the
equator ; while the second satellite and its shadow are higher, the
last being projected on the Great Red Spot.* The row of dark cir-
cular spots represented on the northern, or lower hemisphere, when
they first appeared, had some resemblance to Sun-spots without
a penumbra, with bright markings around them, resembling faculae.
These round spots subsequently enlarged considerably, until they
united along the entire line, encircling the planet, and finally form-
ing a narrow pink belt, which is still visible.

* Note. — By an accidental error in enlarging the original drawing, the satellites and
shadows appear in Plate IX. of double their actual size. The error is one easy of mental
correction.

A S TRONOMICA L DRA WINGS. 83

THE PLANET SATUKN.
PLATE X.

SATURN, which is next to Jupiter in order of distance from the
Sun, while not the largest, is certainly the most beautiful and inter-
esting of all the planets, with his grand and unique system of rings,
and his eight satellites, which, like faithful servants, attend the
planet's interminable journey through space.

Seen with the naked eye, Saturn shines in the night like a star of
the first magnitude, whose dull, soft whiteness is, however, far from
attaining the brilliancy of Venus or Jupiter, although it sometimes
approaches Mars in brightness. Saturn hardly ever exhibits the
phenomenon of scintillation, or twinkling, a peculiarity which
makes it easily distinguishable among the stars and planets of the
heavens.

The synodical period of Saturn occupies I year and 13 days, so
that every 378 days, on an average, this planet holds the same posi-
tion in the sky relatively to the Sun and the Earth.

The mean distance of Saturn from the Sun is a little over
9% times that of our globe, or 872,000,000 miles. Owing to the or-
bital eccentricity, this distance may increase to 921,000,000 miles,
when the planet is in aphelion ; or decrease to 823,000,000 miles,
when it is in perihelion ; Saturn being therefore 98,000,000 miles
nearer to the Sun when in perihelion than in aphelion. If gravita-
tion were free to exert its influence alone, Saturn would fall into
the Sun in 5 years and 2 months.

The distance of Saturn from the Earth varies, according to the
position of the two planets in their respective orbits. At the time
of opposition, when the Earth lies between the Sun and Saturn, this
distance is smallest ; while, on the contrary, at the time of conjunc-
tion, when the Sun lies between the Earth and Saturn, it is great-
est. Owing, however, to the eccentricity of the orbits of Saturn and
our globe, and the inclination of their planes to each other, and
owing also to the variable heliocentric longitude of the perihelion,

84 THE TROUVELOT

the distance of the two planets from each other at their successive
conjunctions and oppositions is rendered extremely variable. At
present it is when the oppositions of Saturn occur in December that
this planet comes nearest to us ; while when the conjunctions take
place in June, the distance of Saturn from the Earth is the greatest
possible. In the former case the distance of the planet from our
globe is only 730,000,000 miles ; while in the last it is 1,014,000,000
miles, the difference between the nearest and farthest points of
Saturn's approach to us being no less than 284,000,000 miles, or over
three times the mean distance of the Sun from the Earth.

From the great variations in the distance of Saturn from the Earth,
necessarily result corresponding changes in the brightness and ap-
parent diameter of this body. When it is farthest from us, its angu-
lar diameter measures but 14" ; while, when it is nearest, it meas-
ures 20".

The orbit of Saturn is inclined 2°3O' to the ecliptic, and its eccen-
tricity, which equals 0,056, is over three times that of the Earth's
orbit.

This planet revolves around the Sun in a period of 29 years and
5^4 months, or 10,759 terrestrial days, which constitutes its sidereal
year. The extension of the immense curve forming the .orbit of this
planet, is no less than 5,505,000,000 miles, which is traversed by
the planet with a mean velocity of a little less than 6 miles per
second, or three times less than the motion of our globe in space.

The real dimensions of the globe of Saturn are not yet known
with accuracy, and the equatorial diameter has been variously esti-
mated by observers, at from 71,000 to 79,000 miles. If we adopt the
mean of these numbers, 75,000 miles, the circumference of the
Saturnian equator would measure 235,620 miles, or g% times the
circumference of our globe ; the surface of Saturn would be 86 times,
and its volume over 810 times that of the Earth.

However great the volume of Saturn, its mass is proportionally
small, being only 90 times greater than that of our globe ; the mean
density of the materials composing this planet being less than that
of cork, and only 0.68 the density of water. The force of gravitation
at the surface of Saturn is greater, by a little over -£-, than it is at
the surface of the Earth ; a body falling in a vacuum at its surface,
would travel 17.59 feet during the first second.

From observations of markings seen on the surface of Saturn, and
from the study of their apparent displacements on the disk, William
Herschel found that the planet rotated upon its axis in loh. i6m.
0.245. Since Herschel's determination, new researches have been

ASTRONOMICAL DRA WINGS. 85

made, and lately, Professor Hall, noticing a bright spot, followed it
for nearly a month, observing its transits across the central meri-
dian of the disk. From these observations he has obtained for the
rotation period loh. 14111. 23.83., a result which agrees very closely with
that obtained 82 years earlier by Herschel, considering the fact that
the markings from which the period of rotation is ascertained are
not fixed on the planet, but are always more or less endowed with
proper motion. The velocity of rotation at the equator is 21,538
miles per hour, or nearly 6 miles per second.

The axis of rotation of Saturn is inclined 64° 18' to the plane of
the orbit, so that its equator makes an angle of 25° 42' with the
same plane. The seasons of this planet therefore present greater
extremes of temperature than those of the Earth, but not quite so
great variations as the seasons of Mars.

The globe of Saturn is not a perfect sphere, but its figure is that
of an oblong spheroid, flattened at the poles. The polar compression
of Saturn is greater than that of any other planet, surpassing even
that of Jupiter. Though not yet determined with a great degree of
accuracy, the compression is known to be between \ and -^ of the
equatorial diameter ; that is, a flattening of about 3, 894 miles, at each
pole, the polar diameter being 7,788 miles shorter than the equa-
torial.

The internal condition of the planet Saturn, whether solid, liquid
or gaseous, cannot be discovered from the examination of its surface,
as its globe is enwrapped in a dense opaque layer of vapors and
cloud-like forms, through which the sight fails to penetrate. The
appearance of this vapory envelope is like that of cumulus clouds,
and one of its characteristics is to arrange itself into alternate bright
and dark parallel belts, broader than those seen on Jupiter, and also
more regular and dark. These belts, which are parallel to the
equator of the planet, vary in curvature with the inclination of its
axis of rotation to the line of sight.

The belts of Saturn, like those of Jupiter, are not permanent, but
keep changing more or less rapidly. Sometimes they have been
observed to be quite numerous ; while at other times they are few.
Occasionally conspicuous white or dark spots are seen on the
surface, although the phenomenon is quite rare. It is from the
observation of such spots that Saturn's period of rotation has been
determined, as stated above. The equatorial zone of Saturn always
appears more white and brilliant than the other parts, as it also ap-
pears more mottled and cloud-like. In late years the globe has
been characterized, and much adorned, by a pale pinkish tint on

86 THE TROUVELOT

its equatorial belt, resembling that of Jupiter, but somewhat fainter.
On either side of the equatorial belt there is a narrower bandr
upon which the mottled appearance is visible. Below these, one
or two dark belts, separated by narrow white bands, are usually
seen ; but, of late, the bands have been less numerous, being re-
placed in high latitudes by a dark segment, which forms a polar
cap to Saturn. The globe of Saturn does not anywhere appear
perfectly white, and when compared with its ring, it looks of a
smoky yellowish tint, which becomes an ashy gray on its shaded
parts. It usually appears darker near the limb than in its central
portions ; although on some occasions I have seen portions of the
limb appear brighter, as if some white spots were traversing it.

Some observers have seen the limb deformed and flattened at
different places, and W. Herschel even thought such a deformation
to be a permanent feature of this globe, which he termed diamond-
shaped, or " square shouldered." But this was evidently an illusion,
since the planet's limb usually appears perfectly elliptical, although
it occasionally appears as if flattened at some points, especially where
it comes in apparent contact with the shadow cast by the globe on
the ring, as observed by myself many times. But with some atten-
tion, it is generally found that this deformation is apparent rather
than real, and is caused by the passage of some large dark spots
over the limb, which is thus rendered indistinguishable from the dark
background upon which it is projected.

What distinguishes Saturn from all known planets, or heavenly
bodies, and makes it unique in our universe, is the marvelous broad
flat ring which encircles its equator at a considerable distance from
it. With a low magnifying power this flat ring appears single, but
when carefully examined with higher powers, it is found to consist of
several distinct concentric rings and zones, all lying nearly in the
same plane with the planet's equator.

At first sight only two concentric rings are recognized, the outer
and the middle, or intermediary, which are separated by a wide and
continuous black line, called the principal division. This line, and
indeed all the features of the surface of the rings are better seen, and
appear more prominent on that part of the ring on either side called
the ansa, or handle. Besides these two conspicuous rings, a third,
of very dark bluish or purplish color, lies between this middle ring,
to which it is contiguous, and the planet. This inner ring, which is
quite wide, is called the gauze or dusky ring. Closer examination
shows that the outer ring is itself divided by a narrow, faint, grayish
line called the pencil line, which, from its extreme faintness, is only

ASTRONOMICAL DRAWINGS. 87

visible on the ansae. Moreover, the middle ring is composed of three
concentric zones, or belts, which, although not* apparently divided
by any interval of space, are distinguished by the different shadings
of the materials composing them. The outer zone of this compound
middle ring is, by far, the brightest of all the system of rings and
belts, especially close to its external border, where, on favorable
occasions, I have seen it appear on the ansae as if mottled over, and
covered throughout with strongly luminous cloud-like masses. On
the ansae of the double outer ring, similar cloudy forms have also
been seen at different times. The second zone of the middle ring is
darker than the first, the innermost being darker still. All the
characteristic points which have thus been described, are shown in
Plate X.

Although suspected in 1838, the dusky ring was not recognized
before 1850, when G. P. Bond discovered it with the 1 5-inch refrac-
tor of the Cambridge Observatory. It was also independently dis-
covered the same year in England by Dawes and Lassell. The
dusky ring differs widely in appearance and in constitution from the
other rings, inasmuch as these last are opaque, and either white or
grayish, while the former is very dark, and yet so transparent that the
. limb of the planet is plainly seen through its substance. On particu-
larly favorable occasions, the appearance of this ring resembles that
of the fine particles of dust floating in a ray of light traversing a dark
chamber. Whatever may be the material of which this ring is com-
posed, it must be quite rarefied, especially towards its inner border,
which appears as if composed of distinct and minute particles of mat-
ter feebly reflecting the solar light. That the inner part of the dusky
ring is composed of separate particles, is proved by the fact that the
part of the ring which is seen in front of the globe of Saturn has its
inner border abruptly deflected and curved inward on entering upon
the disk, causing it to appear considerably narrower than it must be
in reality, a peculiarity which is shown in the Plate. This phe-
nomenon may be attributed to an effect of irradiation, due to the
strong light reflected by the central parts of the ball, which so re-
duces the apparent diameter of the individual particles that they
become invisible to us, especially those near the inner border, which
are more scattered and less numerous than elsewhere.

The dusky ring, which was described by Bond, Lassell and other
astronomers as being equally transparent throughout all its width,
has not been found so by me in later years. The limb of the planet,
seen by these observers through the whole width of the dusky ring
in 1850, could not be traced through its outer half by myself in 1872

88 THE TROUVELOT

and 1874, and this with the very same instrument used by Bond in
his observations of 1848 and 1850. Moreover, I have plainly seen
that its transparency was not everywhere equal, but greatest on the
inner border, from which it gradually decreases, until it becomes
opaque, as proved by the gradual loss of distinctness of the limb,
which vanishes at about the middle of the dusky ring. These facts,
which have been well ascertained, prove that the particles compos-
ing this ring are not permanently located, and are undergoing
changes of relative position. It will be shown that the surface of
the other rings is also subject to changes, which are sometimes very
rapid.

The globe of Saturn is not self-luminous, but opaque. It shines
by the solar light, as is proved by the shadow it casts opposite the
Sun upon the ring. Although receiving its light from the Sun,
Saturn does not exhibit any traces of phases, like the other planets
nearer to the Sun, owing to its great distance from the Earth.
When near its quadratures, however, the limb opposite to the Sun
appears much darker, and shows traces of twilight. As far as can
be ascertained, the rings, with the exception of the inner one, are
opaque, as proved by the strong shadow which they cast on the
globe of Saturn.

The shadows cast by the planet on the ring, and by the ring on
the planet, are very interesting phenomena, inasmuch as they en-
able the astronomer to recognize the form of the surface which re-
ceives them. The shadow cast by the ring on the ball is not quite
so interesting as the other, although it has served to prove that the
surface of this globe is not smooth, as is likewise suggested by its
mottled appearance. I have sometimes found, as have also other
observers, that the outline of this shadow upon the ball was irregu-
lar and indented, an observation which proves either that the sur-
face of the ball is irregular, or that the border of the ring casting
the shadow was jagged. The shadow of the globe on the rings has
much more interest, as it enables us to get at some knowledge of the
form of the surface of the rings, which otherwise is very difficult to
discover, owing to the oblique position in which we always see them.

In general, the shadow of the ball on the middle ring has its out-
line concave towards the planet; while on the outer ring it is usually
slanting, and at a greater distance from the limb than on the mid-
dle, and dusky rings. This form of the shadow evidently proves
that the middle ring stands at a higher level than the two others,
especially towards its outer margin. The system seems to increase
gradually in thickness from the inner border of the dusky ring to the

ASTRONOMICAL DRAWINGS. 89

vicinity of the outer margin of the middle ring, after which it rapidly
diminishes on this border, while the surface of the outer ring is al-
most level.

But this surface is by no means fixed, as its form sometimes
changes, as proved by my observations and those of others. As
may be noticed on Plate X., the outline of the shadow of the planet
on the rings is strongly deviated towards the planet, near the outer
margin of the middle ring; the notch indicating an abrupt change of
level, and a rise of the surface at that point. Some observers have
endeavored to explain these deviations by the phenomena of irradi-
ation, from which it would follow that the maximum effect of de-
viation should be observed where the ring is the brightest, which
does not accord with observation ; as the deepest depression in
the shadow is not to be found usually at the brightest part, which
is towards the outer border of the middle ring, but occurs near its
centre. From these observations it is undoubtedly established that
the surface of the rings is far from being flat throughout, and is,
besides, not permanent, but changes, as would, for instance, the sur-
face of a large mass of clouds seen from the top of a high mountain.
In general, the system is thickest not very far from the outer border
of the intermediary ring.

Some interesting phenomena which I had occasion to observe
before and after the passage of the Sun through the plane of the
rings, on February 6th, 1878, conclusively show that the surface of
this system cannot be of a uniform level, but must be thicker towards
the outer border of the middle ring, thence gradually sloping to-
wards the planet. Many of my observations irresistibly lead to this
conclusion. As it would, however, be out of place to have them
recorded here in detail, I will simply give one of the most character-
istic among them.

From December i8th, 1877, when the Sun was about 41' above
the plane of the rings, to February 6th, 1878, the day of its passage
through their plane, the illuminated surface of this system gradually
decreased in breadth with the lowering of the Sun, until it was lost
sight of, February 5th, on the eve of the passage of the Sun through
their plane. The phenomenon in question consisted in the gradual
invasion of their illuminated surface by what appeared to be a black
shadow, apparently cast by the front part of the outer portion of the
middle ring the nearest to the Sun. On January 25th, when the
elevation of the Sun above the plane of the rings was reduced to 15',
the shadow thus cast had extended so far on their surface that it
reached the shadow cast by the globe on the opposite part of the

90 THE TROUVELOT

ring in the east, and accordingly the remaining portion of the illu-
minated surface of the eastern ansa then appeared entirely discon-
nected from the ball, by a large dark gap, corresponding in breadth
to that of the globe's shadow on the rings. On February 4th, when
the Sun was only 5' above the plane of the rings, their illuminated
and only visible surface was reduced to a mere thread of light, which
on the 5th appeared broken into separate points. It is evident that
the phenomenon was not caused by the obliquity of the ring as seen
from our globe, since the elevation of the Earth above the plane of
the rings — which on December i8th was 3° 20' — was still i° 20' on the
4th of February. In ordinary circumstances, when the Sun is a lit-
tle more elevated, and the rings seen at this last angle, they appear
quite broad and conspicuous, and even the dark open space separating
the dusky ring from the planet is perfectly visible on the ansae, where
the Earth's elevation above their plane is reduced to 40'. It is also
evident that the phenomenon was not to be attributed to the reduc-
tion of the light which they received from the Sun, although the illu-
mination in February might be expected to be comparatively feeble,
since the Sun then shone upon the rings so obliquely; yet (on the sup-
position that their surface is flat) they should have been illuminated
throughout, and if not very brightly, sufficiently so, at least, to make
them visible and as bright as was the narrow thread of light ob-
served on the 4th of February. The phenomenon actually observed
may be explained most readily by assuming, as other phenomena also
indicate, that the surface of the ring is not flat, but more elevated
towards, or in the vicinity of its outer border, from which place it
slopes inwardly towards the planet. On this assumption, it is evi-
dent that the elevated part of the ring the nearest to the Sun would
cast a shadow, which, with the increasing obliquity of the Sun, would
gradually cover the whole surface comprised within the elevated
part, and thus become invisible to us. Several observations made
by Bond and other observers undoubtedly show the same phenome-
non, and do not seem to be intelligible on any other supposition.
From my observations made in 1881 it would appear, however, that
the opposite surfaces of the rings do not exactly correspond in form,
but this may not be a permanent feature, as the surface of this sys-
tem is subject to changes, as already shown.

The dimensions of the rings are great, the diameter of the outer
one being no less than 172,982 miles, the distance from the centre
of the globe to the outer border of the system being, therefore,
86,491 miles. The breadth of the outer ring is 9,941 miles ; that of
the principal division, 2,131 miles ; that of the middle ring, 19,902

ASTRONOMICAL DRA WINGS. 91

miles, and that of the dusky ring, 8,772 miles. The breadth of all
the rings taken together is, therefore, 40,746 miles. The interval
between the surface of Saturn and the inner border of the dusky ring
is 7,843 miles.

The thickness of the system of rings has been variously esti-
mated by astronomers, on account of the great difficulties attending
its determination. While Sir John Herschel estimated it at more
than 250 miles, G. P. Bond reduces it to 40 miles. Both of these
numbers are evidently too small, as so slight a thickness cannot ex-
plain the observed phenomenon of the shadow cast by a portion of
the ring on its own surface, when the Sun is very low in its horizon,
as shown above.

The plane of the system of rings is inclined 27° to the planet's
orbit, and is parallel, or at least very nearly so, with the equator of
the planet, passing, therefore, through its centre, and dividing its
globe into northern and southern hemispheres. Seen from the Earth,
a portion of the ring always appears projected in front of the planet,
thus concealing a small part of its globe, while the opposite portion
passes behind the globe, which hides it from sight.

As the plane of the ring is not affected by the motion of the
planet around the Sun, but always remains parallel to itself, it fol-
lows that as Saturn advances in its orbit the rings must successive-
ly present themselves to us under various angles of inclination, ap-
pearing, therefore, more or less elliptical, and presenting two maxi-
ma and two minima of inclination in the course of one of its revolu-
tions. As the revolution of Saturn is accomplished in 29}^ years,
the maxima and the minima must recur every 14 years and 9
months ; the maxima being separated from the minima by an in-
terval of 7 years and 4% months.

When Saturn arrives at the two opposite points of its orbit,
where the major axis, of its ring is at right angles to the line joining
its centre to that of the Sun, the ring, which is then viewed at an
inclination of 27°, the greatest angle at which it can ever be seen,
has reached its maximum opening, the smaller diameter of its ellipse
being then about half that of the larger. At this moment the out-
er ring projects north and south beyond the globe, which is then
completely enclosed in its ellipse. The maximum opening of the
northern surface of the ring takes place, at present, when Saturn ar-
rives in longitude 262°, in the constellation Sagittarius, and that
of the southern surface when it arrives in longitude 82° in the con-
stellation Taurus. When, on the contrary, Saturn reaches the two
opposite points of its orbit, where the plane of its ring is parallel to

92 THE TROUVELOT

the line joining its centre to that of the Sun, the opening vanishes,
as only the thin edge of the ring is then presented to the Sun and
receives its light, the rest being in darkness. At this moment the
ring disappears, except in the largest telescopes, where it is seen as
an exceedingly thin thread of light ; and the Saturnian globe, hav-
ing apparently lost its ring, appears solitary in the sky, like the other
planets. The disappearance of the ring from this cause occurs now
when Saturn arrives at 90° from either of the positions of maximum
inclination, that is, in longitude 352° in the constellation Pisces, and
in longitude 172° in the constellation Leo.

When the planet is in any other position than one of these last
two, either the northern or the southern surface of the ring is illu-
minated by the Sun, while the opposite surface is in the night, and
does not receive any direct sunlight. At the time of the passage of
the plane of the ring through the Sun's centre, a change takes place
in the illumination of the ring. If it is the northern surface which has
received the rays of the Sun during the previous half of the Saturnian
year, at the moment the plane has passed the centre of the Sun, the
southern surface, after having been buried in darkness for 14^
years, sees the dawn of its long day of the same length. Such a
phenomenon will not occur until 1892, when the passage of the Sun
from the northern to the southern side of the ring will close in twi-
light the day commenced in 1878.

Aside from the periodic disappearance of the ring, resulting from
the passage of the Sun through its plane, the ring may also disap-
pear from other reasons. Just before or just after the time of the
passage of the Sun through the plane of the ring, the Earth and the
Sun may occupy such positions, that while the one is north of the
plane of the ring, the other is south of it, or vice versa, in which
event the ring becomes invisible, because its dark and non-illumi-
nated surface is presented to us. The ring may also become invisi-
ble to us when the Earth passes through its plane.

Since the distance from Saturn to the Sun is to the distance of
the Earth from this last body as 9.54 is to I ; and since the circum-
ference of a circle increases in the same proportion as its radius, it
follows that the diameter of the Earth's orbit projected on the orbit
of Saturn would occupy only ^V part of the latter, or about 12° 2'r
this being 6° i' on either side of the nodes of the rings. To de-
scribe such an arc on its orbit, it takes Saturn almost 360 days
on an average, or almost a complete year ; the Earth describing
therefore almost a whole revolution around the Sun during the time
it takes Saturn to advance 12° 2' on its orbit. Then, when Saturn

ASTRONOMICAL DRAWINGS. 93

occupies a position comprised within an arc 6° i' from either side of
the nodes of its ring, the Earth, by its motion, is liable to encoun-
ter the plane of the ring, when therefore it will only present its thin
edge to us, and becomes invisible. At least one such encounter is
unavoidable within the time during which Saturn occupies either of
these positions on its orbit ; while three frequently happen, and two
are possible.

The natural impression received by looking at the rings, while
seeing the ponderous globe of Saturn enclosed in its interior, is that
this gigantic, but very delicate structure, in order to avoid destruc-
tion, must be endowed with a swift movement of rotation on an
axis perpendicular to its plane, and that the centrifugal force thence
arising counterbalances the powerful attraction of the planet, and
thus keeps the system in equilibrium.

Theoretically, the rotation of the rings is admitted by every
astronomer, as being an essential condition to the existence of the
system, which otherwise, it is thought, would fall upon the planet.
Although the rotation of the rings seems so probable that it is theo-
retically considered as certain, yet its existence has not been satisfac-
torily demonstrated by direct observation, which alone can establish
it on a firm basis as a matter of scientific knowledge.

The determination of the period of rotation of the rings, which is
supposed to be ich. 32m. 153., rests only on the observations of W.
Herschel, made in 1790, from the apparent displacement of irregu-
larities on the ring ; but his results have been contradicted by other
observations, and even by those of Herschel himself, made in later
years.

Although the system of rings is very nearly concentric with the
globe of Saturn, yet the coincidence is not considered as mathemat-
ically exact. It seems to have been satisfactorily demonstrated by
direct observations that the centre of gravity of the system oscillates
around that of the planet, thus describing a minute orbit. This
peculiarity is in accordance with theory, which has shown it to be
essential to the stability of the system.

Besides its system of rings, which makes Saturn the most remarka-
ble planet of the solar system, this globe is attended by eight satel-
lites, moving in orbits whose planes very nearly coincide with the plane
of the rings, except that of the most distant one, which has an inclina-
tion of about 12° 14'. In the order of their distance from the planets,
the satellites of Saturn are as follows : Mimas, Enceladus, Tethys,
Dione, Rhea, Titan, Hyperion and lapetus. The three first satellites-
are nearer to Saturn than the Moon is to the Earth ; while lapetus,

94 THE TROUVELOT

the farthest, is 9^ times the distance of our satellite from us. All
the satellites, with the exception of the farthest, move more rapidly
around Saturn than the Moon moves around the Earth ; while
lapetus, on the contrary, takes almost three times as long to make
one revolution.

The period of revolution of the four inner satellites is accom-
plished in less than three days, that of Mimas being only a little
more than 22 hours. From such swiftness of motion, it is easily
understood how short must be the intervals between the different
phases of these satellites. Mimas, for instance, passes from New
Moon to First Quarter in less than 6 hours.

The distance of the nearest satellite from the planet's surface is
84,000 miles, and its distance from the outer ring only 36,000 miles.
It is difficult to determine the diameter of objects so faint and dis-
tant as are some of these satellites, but the diameter of Titan, the
largest of all, is pretty well known, and estimated to be TV the dia-
meter of the planet, or more than half the diameter of our globe.

lapetus is subject to considerable variations in brilliancy, and as
the maxima and minima always occur when this satellite occupies
the same parts of its orbit, it was conjectured by W. Herschel that,
like our Moon, it turns once upon its axis during each of its revolu-
tions about the planet. It has been shown by my observations,
that lapetus attains its maximum brightness a little before it reaches
its greatest western elongation, and its minimum on the opposite
side.

As the planes of the orbits of the satellites are inclined to the
planet's orbit, it follows that their transits, occultationsand eclipses,
are only possible when Saturn is near its equinoxes. Passages of the
satellites and their shadows across the disk, although rare, have
been observed, and they somewhat resemble the phenomena exhib-
ited by the satellites of Jupiter in transit. When the Earth is very
near the plane of the rings, the satellites, except the farthest,
appear to be in a straight line nearly coincident with the plane of
the rings, and are seen occasionally moving along the thin edge of
the rings, appearing as luminous beads moving on a thread of light.
Owing to the considerable inclination of the axis of rotation of
Saturn to its orbit, the seasons of this planet must have greater ex-
tremes of temperature than those of the Earth. As the year of
Saturn consists of 25,217 Saturnian days, each season, on the aver-
age, is composed of 6,304 Saturnian days.

To an observer on Saturn, the immense arches formed by its
rings would appear as objects of great magnificence, spanning the

ASTRONOMICAL DRA WINGS. 95

sky like soft colorless rainbows. Moreover, the eight moons, several
of which are always visible, would be of the highest interest, with
their swift motions and rapid phases. Mimas, traveling in its orbit
at the rate of 16' of arc per minute of time, moves over a space equal
to the apparent diameter of our Moon in two minutes, or at the rate
of 16° an hour.

Owing to the globular form of Saturn, the rings would be invisi-
ble in latitudes situated above 65° from its equator, and their ap-
parent form and breadth would naturally vary with the latitude.
At 63° only a very small portion of the outer ring would be visible
above the equatorial horizon, where it would appear as a small
segment of a circle. At 62° the principal division would just graze
the horizon. At 46° the outer portion of the dusky ring would
become visible, while at 35° its inner edge would appear above the
horizon. From 65° of latitude down to the equator, the arches of
the rings would be seen more and more elevated above the equato-
rial horizon, but at the same time that they are seen higher up,
their apparent breadth gradually diminishes, owing to the effect of
foreshortening, and at the equator itself the system would only pre-
sent its thin edge to view.

During the summer seasons of either hemisphere of Saturn, the
surface of the rings turned towards such hemisphere, being fully illu-
minated by the Sun, is visible from these regions. In the day time
its light must be feeble and similar to the light reflected by our
Moon during sunshine ; but at night the system would display all its
beauty, and the different rings, with their divisions and their various
reflective powers, must present a magnificent sight.

During the nights of the long winter seasons on Saturn, on the
contrary, the surface of the rings turned towards the hemisphere
undergoing winter, receives no light from the Sun, and is invisible, or
very nearly so, except towards morning and evening, when it may
be faintly illuminated by the secondary light which it receives from
the illuminated globe of Saturn. Although dark and invisible, the
rings may make their form apparent at night by the absence of
stars from the region which they occupy in the sky. Again, in
other seasons, the days present very curious phenomena. In con-
sequence of the diurnal rotation of the planet, the Sun seems to
move in circular arcs, which, owing to the inclination of Saturn's
axis, are more or less elevated above its horizon, according to the
position of the planet in its orbit. As such arcs described by the
Sun in the sky of Saturn are liable to encounter the rings, the Sun
in passing behind them becomes eclipsed. It must be a magnificent

96 THE TROUVELOT

spectacle to witness the gradual disappearance of the fiery globe be-
hind the outer ring, and its early reappearance, but for a moment
only, through the narrow gap of the principal division; to see it van-
ish again behind the middle ring, to reappear a little later through the
semi-transparent dusky ring, but very faint and red colored at first;
and then, gradually brighten up, and finally emerge in all its beauty
from the inner edge of the dusky ring.

It is in latitude 23° that the rings produce the most prolonged
eclipses of the Sun. During a period equivalent to ten of our terres-
trial years, such eclipses continually succeed each other with but
very short periods of interruption; and even during a long series of
rotations of Saturn, the Sun remains completely invisible in those
regions where the apparent arcs which it describes coincide with
the arcs of the rings. In neighboring latitudes, the eclipses of the
Sun, although still frequent, would have a shorter and shorter dura-
tion as the observer should travel north or south. These eclipses
of the Sun must produce a partial darkness of the regions involved
in the shadow of the rings, which may be compared to the darkness
produced on our globe by a total eclipse of the Sun. The frequent
recurrence of these eclipses, and their comparatively long duration
in some regions, must still further reduce the duration of the short
Saturnian days.

The globe of Saturn, as already shown, casts a shadow on the
rings, which, according to the position of the planet in its orbit,
either extends across their whole breadth, or covers only a part of
their surface. The shadow on the rings rising in the east after sun-
set, ascends to the culminating point of their arcs in the sky, in 2h.
34m., and as rapidly descends on the western horizon, to disappear
with sunrise. This shadow, when projected on the rings in the sky,
must be hardly distinguishable from the dark background of the
heavens, except from the absence of stars in the regions which it
occupies. It must appear as a large dark gap, separating the rings
into two parts, and constantly moving from east to west. Possibly
the refraction of the solar rays, in passing through Saturn's atmo-
sphere, may cast some colored light on the rings, similar to that
observed on the Moon during its eclipses.

An observer on the rings would behold phenomena still more
curious, a long day of 14^ years being followed by a long night of
14^ years. The long days of Saturn's rings are, however, diversi-
fied by numerous eclipses of the Sun, which regularly occur every
10^ hours; the phenomenon being due to the interposition of the
globe of Saturn between the rings and the Sun. These eclipses

ASTRONOMICAL DRA WINGS. 97

produce partial obscurations of their surface, lasting from \yz to 2
hours at a time. Although the surface of the rings never receives
direct sunlight during their long nights, yet they are not plunged
all the time in total darkness, as they receive some reflected light
from that part of the globe of Saturn which is illuminated by the
Sun. To the supposed observer on the rings, during every 10^
hours, the immense globe would exhibit continually changing phases.
At first he would see a point of light rapidly ascending from the hori-
zon, and appearing under the form of a half crescent of considerable
radius; 5>^ hours later, the crescent "having gradually increased,
would appear as a half circle, covering ^ of the visible heavens,
its surface being more than 20,000 times as large as the surface of
the Moon. Upon this brilliantly illuminated semi-circle would be
projected the shadows of the rings, appearing as black belts separ-
ated by a narrow luminous band.

It is very difficult for one to conceive how such a delicate struc-
ture, as the system of rings appears to be, can keep together in equi-
librium and avoid destruction from the powerful attraction of the
planet on one side and the disturbing influence of the satellites on
the other. To explain it, several hypotheses have been advanced.
The rings were first supposed to be solid, and upon this supposition
Laplace determined the necessary conditions for their equilibrium; the
most important of which require that the cross section of the rings
should be an ellipse of irregular curvature, and having its major
axis directed towards the centre of the planet, and also that the
system should rotate upon an axis perpendicular to the plane
of the rings. This theory was superseded by another, which sup-
posed the rings to be fluid. This one was soon rejected for a third,
assuming the system to be composed of vapors or gases ; and more
recently, all these theories were considered untenable, and re-
placed by a fourth, which supposes the system of rings to be made
up of a congregation of innumerable small, independent bodies,
revolving around Saturn in concentric zones. Naturally, such a
divergence of opinion can only result from our comparative ignorance
of the subject, and sufficiently indicates our inability to explain the
phenomena ; and it must be admitted that, so far, nothing is cer-
tainly known about this strange system. We shall probably remain
in the same uncertainty until the rotation of the rings is ascertained
by direct observations. It is pretty certain, however, that none of
these theories account for the observed phenomena in their details,
although a partial explanation may be obtained by borrowing some-
thing from each hypothesis.

98 THE TROUVELOT

It has been conjectured, and a theory has been advanced, that
the breadth of the whole ring system is gradually increasing in-
wards, and that it will come in contact with the planet in about
2,150 years ; but the question seems to have been settled in the
negative by the elaborate measurements of the English observers.
It is likely that the increase is only in the defining power of the in-
struments.

ASTRONOMICAL DRAWINGS. 99*

COMETS.

PLATE XL

AMONG the celestial phenomena, none are more interesting than
those mysterious apparitions from the depths which unexpectedly
display their strange forms in our familiar constellations, through
which they wander for a time, until they disappear like phantoms.
A comet, with its luminous diffused head, whence proceeds a long
vapory appendage gradually fading away in the sky, presents an
extraordinary aspect, which may well astonish and deeply impress
the observer. Although these visitors from infinite space do not
now inspire dread, as in by-gone times, yet, owing to the mystery
in which the phenomenon is still involved, the apparition of a large
comet, even in our days, never fails to create a profound sensation,
and in some cases that unconscious fear which results from the
unknown.

The effect of such a spectacle largely depends upon its rarity ;
but since the telescope has been applied to the sounding of the
heavens, it has been found that the appearance of comets is by no
means an unusual occurrence. If so few comets, comparatively, are
seen, it is because most of them are telescopic objects, and are there-
fore invisible to the naked eye. Most of the telescopic comets are
not only too faint to be perceived by the unaided eye, but are insig-
nificant objects, even when observed through the largest telescopes.

It was Kepler's opinion that comets are as numerous in the sky
as fishes are in the ocean. Undoubtedly the number of these bodies
must be great, considering that we can only see them when they
come into the neighborhood of the Earth, and that many even here
remain invisible, or at least pass unperceived. That many of them
have passed unperceived heretofore, is proved by the fact that the
number of those observed becomes greater every year, with the in-
crease of the number of instruments used in their search. The num-
ber of comets observed with the naked eye during historic times
is nearly 600, and that of telescopic comets, which, of course, all

100 THE TROUVELOT

belong to the last few centuries, is more than 200, so that we have
a total number of about 800 comets of which records have been
kept. From theoretical considerations, Lambert and Arago esti-
mated their entire number at several millions, but such speculations
have generally no real value, since they cannot be established on a
firm basis.

Comets remain visible for more or less time, according to their
size and the nature and position of their orbits, but in general, the
large ones can be followed with the telescope for several months
after they have become invisible to the naked eye. The comet of
1861, for example, remained telescopically visible for a year, and
that of 1 8 1 1, for 17 months after disappearing from ordinary sight.

While a comet remains visible, it appears to revolve daily about
us like the stars in general ; but it also moves among the constella-
tions, and from this movement its orbit may be computed like that
of a planet. From the apparent diurnal motion of a comet with the
heavens, result the changes of position which it seems to undergo
in the course of a night. The direction of the head and tail of a
comet, of course, has only changed in regard to the horizon, but
not in regard to the sky, in which they occupy very nearly the same
position throughout a given night, and even for many nights in suc-
cession.

The movements of the comets in their orbits are, like those of
the planets, in accordance with Kepler's laws, the Sun occupying one
of the foci of the orbit they describe ; but the orbits of comets differ,
however, in several points from those of the planets. Their eccen-
tricity is always great, being sometimes apparently infinite, in which
case the orbit is said to be parabolic, or hyperbolic; but the smallness
of the portion of a cometary orbit which can ordinarily be observed,
makes it difficult to determine this with certainty. Again, while the
planetary orbits are usually near the plane of the ecliptic, those of
comets frequently have great inclinations to that plane, and even
when the inclination is less than 90°, the comet may have a retro-
grade movement, or, in other words, a movement contrary to the
course in which all the planets revolve about the Sun.

Notwithstanding these differences between the elements of the
orbits of the comets and those of the planets, the fact that each has
the Sun in one focus indicates that the body moving in it is a mem-
ber of the solar system, either for the time, or permanently, accord-
ing to the nature of its orbit.

A distinction may accordingly be made between the comets
which are permanent members of our solar system and those which

ASTRONOMICAL DRAWINGS. 101

are only accidental or temporary visitors. Those moving in ellipti-
cal orbits around the Sun, like the planets, and therefore having a
determinate period of revolution, from which the time of their suc-
cessive returns may be predicted, are permanent members of our
system, and are called periodic comets. All comets moving in para-
bolical or hyperbolical curves, are only temporary members of the
solar system, being apparently strangers who have been diverted
from their courses by some disturbing influence. No comet is classed
as periodical which does not follow a perceptibly elliptical orbit. Any
comet passing around the Sun at the mean distance of the Earth
from this body, with a velocity of 26 miles per second, will fly off
into infinite space, to return to us no more.

The time of revolution of the different periodic comets thus far
observed varies greatly, as do also the distances to which they re-
cede from the Sun at aphelion. Whilst the period of revolution of
Encke's comet, the shortest thus far known, is only 3^2 years, that of
the comet of 1844, II., is 102,000 years ; and whilst the orbit of the
first is comprised within the orbit of Jupiter, that of the last extends
to a distance equal to 147 times the distance of Neptune from the
Sun. But so vast an orbit cannot be accurately determined from
the imperfect data at our disposal.

The periodic comets are usually divided into two classes. The
comets whose orbits are within the orbit of Neptune are called in-
terior comets, while those whose orbits extend beyond that of Nep-
tune are called exterior comets. The known interior periodic com-
ets are twelve in number, while, including all the cases in which
there is some slight evidence of elliptic motion, the number of ex-
terior comets observed is six or seven times as great. The periodic
comets of short period are very interesting objects, inasmuch as by
their successive returns they afford an opportunity to calculate their
motions and to observe the physical changes which they undergo
in their intervals of absence.

From observation of the periodic comets, it has been learned
that the same comet never presents twice the same physical appear-
ance at its different returns, its size, shape and brilliancy varying so
greatly that a comet can never be identified by its physical charac-
ters alone. It is only when its elements have been calculated, and
are found to agree with those of a cometary orbit previously known,
that the two comets can be identified one with the other. There
are reasons to believe that, in general, comets decrease in bright-
ness and size at each of their successive returns, and that they are
also continually losing some of their matter as they traverse their
orbits.

102 THE TROUVELOT

When very far away from us, all comets appear nearly alike, con-
sisting of a faint nebulosity, of varying dimensions. When a comet
first appears in the depths of space, and travels towards the Sun, it
generally resembles a faint, uniformly luminous nebulosity, either
circular or slightly elongated in form. As it approaches nearer to
the Sun, a slight condensation of light appears towards its centre,
and as it draws still nearer, it becomes brighter and brighter, and in
condensing forms a kind of diffused luminous nucleus. At the same
time that the comet acquires this concentration of light, the nebu-
losity gradually becomes elongated in the direction of the Sun.
These effects generally go on increasing so long as the comet is
approaching the Sun ; the condensation of light sometimes forms a
bright nucleus, comparable to a very brilliant star, while the elonga-
tion becomes an immense appendage or tail. When the comet has
passed its perihelion and recedes from the Sun, the inverse phenom-
ena are observed ; the comet, decreasing in brightness, gradually
loses its nucleus and tail, resumes its nebulous aspect, and finally
vanishes in space, to appear again in due course, if it chance to be a
periodic comet. While all comets become brighter in approaching
the Sun, they do not all, however, develop a large tail, some of
them showing only a slight elongation.

When a comet is first discovered with the telescope at a great
distance from the Sun, it is difficult to predict whether it will be-
come visible to the naked eye, or will remain a telescopic object, as
it is only in approaching the Sun that these singular bodies acquire
their full development. Thus, Donati's comet, whose tail became
so conspicuous an object at its full appearance in 1858, remained two
months after its discovery by the telescope without any indication
of a tail. The comet of Halley, which before and after its return in
1759, remained five years inside of the orbit of Saturn, showed not
the least trace of its presence during the greater part of this time.
Nothing but calculation could then indicate the position in the sky
of this invisible object, which was so prominent when it approached
the Sun.

Another curious phenomenon exhibited by comets, and first no-
ticed by Valz, is that in approaching the Sun the nebulosity com-
posing these bodies contracts, instead of dilating, as would be
naturally supposed from the greater amount of solar heat -which
they must then receive. In receding from the Sun, on the contrary,
they expand gradually. As comets approach the Sun, the tail and
nucleus are developed, while the nebulosity originally constituting
these comets contracts, as if its material had been partly consumed

ASTRONOMICAL DRAWINGS. 103

in this development. In a certain sense it may be said that the
comets are partly created by the Sun; in more exact terms, the
changes of form which they undergo are induced by the Sun's action
upon them at different distances and under varying conditions.
Moreover, they are rendered visible by its influence, without which
they would pass unperceived in our sky. When a comet disappears
from view, it is not because its apparent diameter is so much reduced
by the distance that it vanishes, but rather on account of the dim-
inution of its light, both that which it receives from the Sun, and
its own light; these bodies being in some degree self-luminous, as
will be shown below.

The large comets, such as can be seen with the naked eye, al-
ways show the following characteristics, on examination with the
telescope. A condensation of light resembling a diffused star forms
the brightest part of the comet, this condensation being situated
towards the extremity the nearest to the Sun. It is this starlike
object which is called the nucleus. The nucleus seems to be entirely
enclosed in a luminous vapory envelope of the same general texture,
called the coma. This envelope, which is quite variable in bright-
ness and form, is brightest next to the nucleus, and gradually fades
away as it recedes from it. The nucleus and the coma, considered as
a whole, constitute the head of a comet. From the head of a comet
proceeds a long trail of pale nebulous light, which usually grows
wider, but fainter, as it recedes from the nucleus, and insensibly
vanishes in the sky. This delicate appendage, or tail, as it is com-
monly called, varies very much in size and shape, not only in differ-
ent comets, but in the very same comet, at different times. Its
direction is generally opposite to that of the Sun from the head of
the comet.

The nuclei vary very much in brightness, in size and in shape;
and while in some telescopic comets they are either absent or barely
distinguishable as a small condensation of light, in bright comets
they may become plainly visible to the naked eye, and they some-
times even surpass in brightness the most brilliant stars of the
heavens. But whatever may be the size of cometary nuclei, they
are subject to sudden and rapid changes, and vary from day to day.
Sometimes they appear exceedingly brilliant and sharply outlined,
while at other times they are so dim and diffused that they are hardly
distinguishable from the coma of which they seem then to form a part.

From my observations upon the comets which have appeared
since the year 1873, it is apparent that the changes in the nucleus,
coma and tail, are due to a solar action, which contracts or expands

104 THE TROUVELOT

these objects in such a manner that the nuclei become either bright
and star-like, or dim and diffused, in a very short time. I had ex-
cellent opportunity, especially in the two large comets of 1881, to
observe some of these curious changes, a description of which will
give an idea of their extent and rapidity. On July 2d, 1881, at 9
o'clock, the nucleus of comet 1881, III., which is represented on Plate
XI., appeared sharply defined, bright and considerably flattened cross-
wise; but half an hour later it had considerably enlarged and had
become so diffused that it could hardly be distinguished from the
coma, with which it gradually blended. It is perhaps worth mention
that, at the time this last observation was made, an aurora borealis
was visible. This comet 1881, III., underwent other very import-
ant changes of its nucleus, coma and tail. On June 25th, the nucleus,
which was bright and clearly defined, was ornamented with four
bright diverging conical wing's of light, as shown on Plate XI. On
the 26th these luminous wings had gone, and the nucleus appeared
one-third smaller. On the 28th it had enlarged, but on the 2Qth
its shape was considerably altered, the nucleus extending in one
direction to three or four times its diameter on previous nights, and
being curved, so as to resemble a comma. On the 6th of July the
nucleus of this comet showed the greatest disturbances. The nucleus,
which had appeared perfectly round on the evening of the 5th, was
found much elongated at 10 o'clock on the 6th, forming then a
straight, acute, and well-defined wedge of light, inclined upwards to
the left. The length of the nucleus, at this time, was three or four
times its ordinary diameter. At the same time rapid changes oc-
curred ; the strangely shaped nucleus soon became unsteady, ex-
tending and contracting alternately, and varying greatly in bright-
ness. At ich. 45m., the elongated nucleus, then gently curved,
took the shape of a succession of luminous knots, which at times
became so brilliant and distinct that they seemed to be about to
divide and form separate nuclei; but such a separation did not actu-
ally occur, at least while I was observing. While these important
changes were going on in the comet, a bright auroral arch appeared
in the north, which lasted only a short time. On July /th, the sky
being cloudy, no observations were made, but on the 8th I observed
the comet again. The nucleus had then resumed its circular form, but
it was yet very unsteady, being sometimes small, bright and sharp,
while a few seconds later it appeared twice as large, but dim in out-
lines; and sometimes an ill-defined secondary nucleus appeared at
its centre. On several occasions the nucleus appeared as if it were
double, one nucleus being apparently projected partly upon the other.

ASTRONOMICAL DRAWINGS. 105

The nuclei of comets are sometimes very small, and in other
cases very large. Among those which have been measured, the
nucleus of the comet of 1798, L, was only 28 miles in diameter, but
that of Donati's comet, in 1858, was 5,600 miles, and that of the
comet of 1845 was 8,000 miles in diameter.

The coma of comets is found to be even more variable than the
nucleus. The changes observed in the coma are generally in close
connection with those of the nucleus and tail, the same perturba-
tions affecting simultaneously the whole comet. While the coma
of the comet of 1847 was only 18,000 miles in diameter, that of
Halley's comet, in 1835, was 357,000 miles, and that of the comet of
1811 was 1,125,000 miles in diameter. In general, as already stated,
the coma of a comet decreases in size in approaching the Sun. That
of Encke's comet, which, on October Qth, 1838, had a diameter of
281,000 miles, gradually decreased at a daily mean rate of 4,088 miles
in going towards the Sun ; so that, on December I7th, when the
distance of the cornet from the Sun was more than four times less
than it was on the first date, its diameter was reduced to 3,000
miles.

The form of the coma, in that part which is free from the tail, is
in general a portion of a circle, but is sometimes irregular, with
its border deformed. Thus, the border of the coma of Halley's
comet was depressed at one point towards the Sun. I observed a
similar phenomenon in Coggia's comet, with the great refractor of
the Harvard College Observatory, on July I3th, 1874, when its
border appeared deeply depressed on the side nearest to the Sun, as
if repelled by this body. The coma of comet 1881, III., showed also
very sigular outlines on the nights of the 25th and 26th of June,
when its border was so deeply depressed that the coma appeared as
if it were double. Luminous rays and jets often radiate from the
nucleus across the coma, and describe graceful lateral curves, falling
backwards and gradually fading away into the tail, of which they
then form a part. The rays and jets emitted by the nucleus seem at
first to obey the solar attraction and travel towards the Sun ; but
they are soon repelled, and move backward towards the tail. It is a
mystery, as yet unexplained, how these cometary jets, which at
first seem to obey to the laws of attraction, are compelled to re-
treat apparently by superior opposing forces. Among the forces
of nature, we know of no other than those of an electrical sort, which
would act in a similar manner ; but this explanation would require
us to assume some direct electrical communication between the
comet and the Sun. Considering the distance between the two

106 THE TROUVELOT

bodies, and the probable absence or great tenuity of the gaseous
material in interstellar space, such an assumption is a difficult one.

Under the action of the solar forces, the c.oma also very fre-
quently forms itself into concentric luminous arcs, separated by
comparatively dark intervals. These luminous semi-circles vary in
number, but sometimes there are as many as four or five at a time.
All great comets show these concentric curves more or less, but
sometimes only a portion is visible, the rest of the coma having
a different structure. When great comets approach near the Sun,
their coma is generally composed of two distinct parts, an inner and
an outer coma, the inner one being due to the luminous jets issu-
ing from the nucleus, which, never extending very far, form a dis-
tinct, bright zone within the fainter exterior coma.

The tails of comets, which are in fact a prolongation of the comar
are likewise extremely variable in form. They are sometimes straight
like a rod; again, are curved like a sabre, or even crooked like an S,
as was that of the comet of 1769. They are also fan-shaped, point-
ed, or of the same width throughout. Many of these appendages
appear longitudinally divided through their middle by a narrow,
darkish rift, extending from the nucleus to the extremity. This
peculiarity appears in the comet shown on Plate XI. Sometimes
the dark rift does not commence near the nucleus, but at some
distance from it, as I observed in the case of comet 1881, III., on
June 26th. This dark rift is not a permanent feature of a comet's
tail, but may be visible one day and not at all the next. Comet
1 88 1, III., which had shown a dark rift towards the end of June, did
not exhibit any such rift during July and August, when, on the
contrary, its tail appeared brighter in the middle. Coggia's comet,
which showed so prominent a dark rift in July, 1874, had none on
June loth. On the contrary, the tail was on that date very bright
along its middle, as also along each of its edges.

The tail of a comet does not invariably point directly away from
the Sun, as above mentioned, and sometimes the deviation is con-
siderable ; for instance, the tail of the comet of 1577 deviated 21°
from the point opposite to the Sun.

In general, the tail inclines its extremity towards the regions of
space which it has just left, always presenting its convex border to
the regions towards which it is moving. It is also a remarkable
fact that this convex border, moving first in space, always appears
brighter and sharper than the opposite one, which is often diffused.
From these peculiarities it would seem that in moving about the
Sun the comets encounter some resistance to their motion, from the

ASTRONOMICAL DRAWINGS. 107

medium through which they pass, and that this resistance is suffi-
cient to curve their tails away from the course in which they move,
and to crowd their particles together on the forward side. It is es-
pecially when they approach their perihelion, and move more rapidly
on a curve of a shorter radius, that the comets' tails show the greatest
curvature, unless their position in regard to the observer prevents
their being advantageously seen. The tail of Donati's comet pre-
sented a fair illustration of this peculiarity, its curvature having aug-
mented with the velocity of the comet's motion about the Sun. But
possibly this phenomenon has another cause, and may be found
rather in the solar repulsion which acts on comets and is not instan-
taneously propagated throughout their mass.

Although, in general, comets have but one tail, it is not very
rare to see them with multiple tails. The comets of 1807 and 1843
had each a double tail; Donati's comet, in 1858, showed several nar-
row, long rectilinear rays, issuing from its abruptly curved tail. The
comet of 1825 had five branches, while that of 1744 exhibited no
less than six distinct tails diverging from the coma at various
angles. In general character the multiple and single tails are simi-
lar. When a comet has two tails, it is not rare for the second to
extend in the general direction of the Sun, as was the case with
the great comet of 1881, III., represented on Plate XL From July
I4th to the 2 1st it exhibited quite an extended conical tail, starting
obliquely downwards from the right side of the coma, and directed
towards the Sun. From the 24th of July to the 2d of August this
secondary tail was exactly opposite in its direction from that of the
primary tail, and gave to the head a very elongated appearance.
Comet 1 88 1, IV., also exhibited a secondary appendage, not directed
towards the Sun, but making an angle of about 45° with the main tail.

These cometary appendages sometimes attain prodigious dimen-
sions. The comets of 1680 and 1769 had tails so extended that,
after their heads had set under the horizon, the extremities of these
immense appendages were still seen as far up as the zenith. In a sin-
gle day the tail of the comet of 1843 extended 100°, and it was
thrust from the comet " as a dart of light " to the enormous distance
of 48,500,000 miles, and yet of this immense appendage nothing was
left on the following day. The tail of Donati's comet, in 1858, at-
tained a real length of 42,000,000 miles, while that of the great
comet of 1843 nad the enormous length of 200,000,000 miles. If
this last comet had occupied the position of the Sun, which it ap-
proached very nearly for a moment, the extremity of its tail would
have extended 60,000,000 miles beyond the orbit of Mars.

108 THE TROUVELOT

In some cases the tails of comets have been seen undulating and
vibrating in a manner similar to the undulations and coruscations of
light characteristic of some auroras. Many observers report having
seen such phenomena. The comet of 1769 was traversed by lumin-
ous waves and pulsations, comparable to those seen in the aurora
borealis. I myself observed these curious undulations in Coggia's
comet in 1874, while the head of this object was below the horizon.
For an hour the undulations rapidly succeeded each other, and ran
along the whole length of the tail.

Some of the brightest comets have shone with such splendor that
they could be observed easily in full sunshine. Many comets, such
as those of 1577 and 1744, have equaled Sirius and Venus in bril-
liancy. The great comet of 1843, which suddenly appeared in our
sky, was so brilliant that it was seen by many observers at noon
time, within a few degrees from the Sun. I remember that I myself
saw this remarkable object .in the day time, with a number of per-
sons, who were gazing at the wonderful apparition. So brilliant was
this comet, that besides its nucleus and head, a portion of its tail
was also visible in the day time, provided the observer screened
his eyes from the full sunlight by standing in the shadow of some
building.

Of all the bodies revolving around the Sun, none have been
known to approach so near its surface as did the comet of 1843.
When it arrived at perihelion, the distance from the centre of its
nucleus to the surface of the Sun's photosphere was only 96,000
miles, while the distance from surface to surface was less than 60,000
miles. This comet, then, went through the solar atmosphere, and
in traversing it with its tremendous velocity of 366 miles per second,
may very possibly have swept through some solar protuberances,
many of which attain much higher elevations than that at which
the comet passed. The comet of 1680 also approached quite near
the surface of the Sun, and near enough to encounter some of the
high solar protuberances, its distance at perihelion being about two-
thirds of the Moon's distance from the Earth. The rapidity of motion
of the comet of 1843 was such, when it approached the Sun, that it
swept through all that part of its orbit which is situated north of the
plane of the ecliptic in a little more than two hours, moving in this
short time from one noSe to the other, or 1 80°.

But if some comets have a very short perihelion distance, that
of others is considerable. Such a comet was that of 1729, whose
perihelion distance was 383,000,000 miles, the perihelion point being
situated between the orbits of Mars and Jupiter.

ASTRONOMICAL DRA WINGS. 109

While some comets come near enough to the Sun at perihelion
to be volatilized by its intense heat, others recede so far from it at
.aphelion that they may be said to be frozen. The shortest comet-
ary aphelion distance known is that of Encke's comet, whose great-
est distance from the sun is 388,000,000 miles. But that of the
•comet of 1844 is 406,000,000,000 miles from the Sun. The comets
-of 1863 and 1864 are so remote in space when they reach their
aphelion points that light, with its velocity of 185,500 miles a sec-
ond, would require 171 days in the first case, and 230 in the last, to
pass from them to the Earth.

The period of revolution of different comets also varies immense-
ly. While that of Encke's comet is only 3^ years, that of comet
1864, II., is 280,000 years.

Among the periodic comets of short period, some have exhibited
highly interesting phenomena. Encke's comet, discovered in 1818,
is remarkable for the fact that its period of revolution diminishes at
each of its successive returns, and consequently this comet, with each
revolution, approaches nearer and nearer to the Sun. The decrease
•of the period is about 2^/2, hours at each return. Although the de-
crease is small, if it go on in future as it does at present, the inevi-
table consequence will be that this comet will finally fall into the
Sun. This curious phenomenon of retardation has been attributed
by astronomers to the existence of a resisting medium filling space,
but so rare and ethereal that it does not produce any sensible effect
•on the movements of the planets. But some other causes may re-
tard this comet, as similar retardations have not been observed in the
case of other periodic comets of short period. These, however, are
not so near to the Sun, and perhaps our luminary may be surround-
ed by matter of extreme tenuity, which does not exist at a greater
distance from it.

Another of the periodic comets which has exhibited a very re-
markable phenomenon of transformation is Biela's comet, which di-
vided into two distinct parts, moving together in the same direc-
tion. When this comet was first detected at its return in 1845, it
presented nothing unusual, but in the early part of 1846 it was
noticed by several astronomers to be divided into two parts of un-
equal brightness, forming thus a twin comet. At its next return in
1852, the two sister comets were still traveling in company, but
their distance apart, which in 1846 was 157,000 miles, had increased
to 1,500,000 miles. • At the two next returns in 1859 and 1865, their
position not being very favorably situated for observation, the
•comets were not seen. In 1872 the position should have been favor-

110 THE TROUVELOT

able for observation, and they were consequently searched for, but
in vain ; neither comet was found. An astronomer in the southern
hemisphere, however, found a comet on the track of Biela's, but cal-
culation has shown that the two objects are probably not identical,
since this comet was two months behind the computed position for
Biela's. It will be shown in the following chapter that our globe
probably crossed the orbit of Biela's comet on November 2/th, 1872,
and the phenomena resulting from this passage will be there de-
scribed.

It is seen from these observations that comets may be lost or
dissipated in space by causes entirely unknown to us. Biela's comet
is not the only one which has been thus disintegrated. Ancient
historians speak of the separation of large comets into two or more
parts. In 1661 Hevelius observed the apparent division of the comet
of that year and its reduction to fragments. The return of this
comet, calculated for 1790, was vainly waited for ; the comet was
not seen.

Other comets, whose periods of revolution were well known,
have disappeared, probably never to return. Such is Lexell's
comet, whose period was 5^ years ; also De Vice's comet,
both of which are now lost. It is supposed that Lexell's comet,
which passed twice very near the giant planet Jupiter, had its or-
bit changed from an ellipse to a parabola, by the powerful disturb-
ing influence of this planet, and was thus lost from our system.
Several other comets, in traveling over their different orbits, have
approached near enough to Saturn, Jupiter and the Earth to have
their orbits decidedly altered by the powerful attraction of these
bodies.

But since comets are liable to pass near the planets, and several
have orbits which approach that of the Earth, it becomes important
for us to know whether an encounter of such a body with our globe
is possible, and what would then be the result for us. Although
that knowledge would not enable us to modify the possibilities of
an encounter, yet it is better to know the dangers of our naviga-
tion through space than to ignore them. This question of a collision
of the Earth with a comet has been answered in different ways, ac-
cording to the ideas entertained in regard to the mass of these
bodies. While some have predicted calamities of all kinds, such as-
deluges, conflagrations, or the reduction of the Earth to incandes-
cent gases, others have asserted that it would produce no more
effect than does a fly on encountering a railroad train. In our days-
astronomers entertain very little fears from such an encounter, be-

ASTRONOMICAL DRAWINGS. Ill

cause the probabilities of danger from an occurrence of this sort are
very slight, the mass of an ordinary comet being so small com-
pared with that of our globe. We know with certainty that the
Earth has never had an encounter with a comet by which it has
been transformed into gases, at least within the several millions of
years during which animal and vegetable life have left their marks
upon the stony pages of its history, otherwise these marks would
not now be seen. If, then, such an accident has not happened dur-
ing this long period, the chances for its occurring must be very
small, so small indeed that they might almost be left out of the
question. It is true that our globe shows signs of great perturba-
tions of its surface, but we have not the slightest proofs that they
resulted from an encounter with a celestial body. It seems very
probable that our globe passed through the tail of the comet of
1861, before it was first seen on June 2Qth ; but nothing unusual
was observed, except perhaps some phosphorescent light in the at-
mosphere, which was afterwards attributed to this cause.

The density and mass of comets must be comparatively very
small. Their tails consist of matter of such extreme tenuity that it
affects but very little the light of the small stars over which they
pass. The coma and nucleus, however, are not quite so transparent,
and may have greater masses. On several occasions I have seen the
light of stars reduced by the interposition of cometary matter, comet
1881, III., presenting remarkable cases of this sort. On July 8th, at
loh. 5om., several small stars were involved in this comet, one of
which passed quite near the nucleus through the bright inner coma.
At that time the comet was greatly disturbed, its nucleus was con-
tracting and enlarging rapidly, and becoming bright and again faint
in an instant. Every time that the nucleus grew larger, the star be-
came invisible, but reappeared the moment the nucleus was reduced
in size. This phenomenon could not be attributed to an atmospheric
effect, since, while the nucleus was enlarging, a very small inner
nucleus was visible within the large diffused one, the matter of which
had apparently spread over the part of the coma in which the star
was involved, making it invisible.

That the mass of comets is small, is proved by the fact that they
have sometimes passed near the planets without disturbing them in
any sensible manner. Lexell's comet, which in 1770 remained four
months very near Jupiter, did not affect in the least the orbits, or
the motions of its satellites. The same comet also came within less
than 1,500,000 miles from the Earth, and on this occasion it was cal-
culated that its mass could not have been the 3TFVv Part of that of

112 THE TROUVELOT

our globe, since otherwise the perturbations which it would have
caused in the elements of the Earth's orbit would have been sensi-
ble. There was, however, no change. If this comet's mass had
been equal to that of our globe, the length of our year would have
been increased by 2h. 47m. The comet of 1837 remained four days
within 3,500,000 miles of the Earth, with no sensible effect.

It seems quite difficult to admit that the denser part of a comet
forming the nucleus is solid, as supposed by some physicists, since
it is so rapidly contracted and dilated by the solar forces, while the
comet is yet at a too great distance from the Sun to allow these
effects to be attributed to solar heat alone. This part of a comet,
as indeed the other parts, seems rather to be in the gaseous than in
the solid state ; the changes observed in the intensity of its light
and in its structure may be conceived as due to some solar action
partaking of the nature of electricity.

It has been a question whether comets are self-luminous, or
whether they simply reflect the solar light. When their light is
analyzed by the spectroscope, it is found that the nucleus of a comet
generally gives a continuous spectrum, while the coma and tail give
a spectrum consisting of several bright diffused bands. The spec-
trum given by the nucleus is rarely bright enough to allow the dark
lines of the solar spectrum to be discerned upon it ; but such lines
were reported in the spectrum of comet 1881, III., a fact proving
that this nucleus at least reflected some solar light. The nucleus of
a comet may be partly self-luminous, and either solid, liquid, or com-
posed of incandescent gases submitted to a great pressure. As to
the coma and tail, they are evidently gaseous, and partly, if not en-
tirely, self-luminous, as is proved by the band spectrum which they
give. The position of these bands, moreover, indicates that the lu-
minous gases of which they are composed contain carbon. The
phenomena of polarization, however, seem to prove that these parts
of comets also reflect some solar light.

No theory so far proposed, to explain comets and the strange
phenomena they exhibit, seems to have been successful in its
attempts, and the mystery in which these bodies have been involved
from the beginning of their apparition, seems to be now nearly
as great as ever. It has been supposed that their tails have no
real existence, but are due to an optical illusion. Prof. Tyndall has
endeavored to explain cometary phenomena by supposing these
bodies to be composed of vapors subject to decomposition by the
solar radiations, and thus made visible, the head and tail being an
actinic cloud due to such decompositions. According to this view,

ASTRONOMICAL DRAWINGS. 118

the tails of comets would not consist of matter projected into space,
but simply of matter precipitated by the solar rays in traversing
the cometary nebulosity. The endeavor has also been made to ex-
plain the various phenomena presented by comets by an electrical
action of the Sun on the gases composing these objects. Theories
taking this as a base seem to us to be more likely to lead to valuable
results. M. Faye, who has devoted much time and learning to this
subject, assumes a real repulsive force of the Sun, acting inversely
to the square of the distance and proportionally to the surface, and
not to the mass as attraction does. He supposes, however, that
this repulsive force is generated by the solar heat, and not by elec-
tricity. Prof. Wm. Harkness says that many circumstances seem to
indicate that the comets' tails are due, in a great measure, to elec-
trical phenomena.

The fact that the tails of comets are better defined and brighter
on the forward side, associated with the other fact that they curve
the most when their motion is most rapid, sufficiently indicates that
these appendages are material, and that they either encounter some
resistance from the medium in which they move, or from a solar re-
pulsion. The phenomena of condensation and extension, which I
have observed in the comets of 1874 and 1881, added to the curious
behavior exhibited by the jets issuing from the nucleus, seem to in-
dicate the action of electrical forces rather than of heat. The main
difficulty encountered in the framing of a theory of comets consists
in explaining how so delicate and extended objects as their tails
seem to be, can be transported and whirled around the Sun at their
perihelion with such an enormous velocity, always keeping opposite
to the Sun, and, as expressed by Sir John Herschel, " in defiance of
the law of gravitation, nay, even of the received laws of motion."

To consider the direction of the comets' tails as an indirect effect
of attraction, seems out of the question ; the phenomenon of repul-
sion so plainly exhibited by these objects seems to point to a posi-
tive solar repulsion, as alone competent to produce these great
changes. The repulsive action of the Sun on comets' tails might be
conceived, for instance, as acting in a manner similar to that of a
powerful current of wind starting from the Sun, and constantly chang-
ing in direction, but always keeping on a line with the comet. Such
a current, acting on a comet's tail as if it were a pennant, would
drive it behind the nucleus just as observed. If it could once be
ascertained that the great disturbances on comets correspond with
the magnetic disturbances on our globe and with the display of the
auroral light, the electric nature of the forces acting so strangely on

114 THE TROUVELOT

the comets would be substantially demonstrated. I have shown
that some of the great disturbances observed in the comets of 1874
and 1 88 1 have coincided with auroral displays, and it will be shown
hereafter that similar displays have also coincided with the passage
of meteoric showers through our atmosphere. Whether these simul-
taneous phenomena were simple coincidences having no connec-
tion, or whether they are the result of a common cause, can only
be ascertained by long continued future observations.

ASTRONOMICAL DRAWINGS. 115

SHOOTING-STARS AND METEORS.
PLATE XII.

WHILE contemplating the heavens on a clear moonless night,
we occasionally witness the sudden blazing forth of a star-like
meteor, which glides swiftly and silently across some of the constel-
lations, and as suddenly disappears, leaving sometimes along its
track a phosphorescent trail, which remains visible for a while and
gradually vanishes. These strange apparitions of the night are
called Falling or Shooting-stars.

There is certainly no clear night throughout the year during
which some of these meteors do not make their appearance, but
their number is quite variable. In ordinary nights only four or five
will be observed by a single person in the course of an hour ; but on
others they are so numerous that it becomes impossible to count
them. When the falling stars are only a few in number, and appear
scattered in the sky, they are called Sporadic Meteors, and when
they appear in great numbers they constitute Meteoric Showers or
Swarms.

Probably there is no celestial phenomenon more impressive than
are these wonderful pyrotechnic displays, during which the heavens
seem to break open and give passage to fiery showers, whose lumin-
ous drops describe fantastic hieroglyphics in the sky. While observ-
ing them, one can fully realize the terror with which they have
sometimes filled beholders, to whom it seemed that the stability of
the universe had come to an end, and that all the stars of the
firmament were pouring down upon the Earth in deluges of fire.

The ancients have left record of many great meteoric displays,
and the manner in which they describe them sufficiently indicates
the fear caused by these mysterious objects. Among the many
meteoric showers recorded by ancient historians may be mentioned
one observed in Constantinople, in the month of November, 472,
when all the sky appeared as if on fire with meteors. In the year 599,

116 THE TROUVELOT

meteors were seen on a certain night flying in all directions like
fiery grasshoppers, and giving much alarm to the people. In March,
763, " the stars fell suddenly, and in such crowded number that
people were much frightened, and believed the end of the world had
come." On April loth, 1095, the stars fell in such enormous quan-
tity from midnight till morning that they were as crowded as are
the hail stones during a severe storm.

In modern times the fall of the shooting-stars in great number
has been frequently recorded. One of the most remarkable mete-
oric showers of the eighteenth century occurred on the night of
November I3th, 1799, and was observed throughout North and South
America and Europe. On this memorable night thousands of fall-
ing stars were seen traversing the sky between midnight and morn-
ing. Humboldt and Boupland, then traveling in South America,
observed the phenomena at Cumana, between two and five o'clock
in the morning. They saw an innumerable number of shooting-
stars going from north to south, appearing like brilliant fire-works.
Several of these meteors left long phosphorescent trails in the sky,
and had nuclei whose apparent diameter, in some cases, surpassed
that of the Moon.

The shower of November I3th, 1833, was still more remarkable
for the great number of meteors which traversed the heavens, and
was visible over the whole of North and South America. On that
occasion the falling stars were far too numerous to be counted, and
they fell so thickly that Prof. Olmsted, of New Haven, who observed
them carefully, compared their number at the moment of their maxi-
mum fall to half that of the flakes of snow falling during a heavy
storm. This observer estimated at 240,000 the number of meteors
which must have traversed the heavens above the horizon during
the seven hours while the display was visible.

In the years 1866, 1867 and 1868, there were also extraordinary
meteoric displays on the night of November I3th. It was on the
last mentioned date that I had the opportunity to observe the re-
markable shower of shooting-stars of which I have attempted to
represent all the characteristic points in Plate XII. My observa-
tions were begun a little after midnight, and continued without in-
terruption till sun-rise. Over three thousand meteors were observed
during this interval of time in the part of the sky visible from a
northern window of my house. The maximum fall occurred between
four and five o'clock, when they appeared at a mean rate of 15 in a
minute.

In general, the falling stars were quite large, many being supe-

ASTRONOMICAL DRA WINGS. 117

rior to Jupiter in brightness and apparent size, while a few even sur-
passed Venus, and were so brilliant that opaque objects cast a strong
shadow during their flight. A great many left behind them a lumin-
ous train, which remained visible for more or less time after the
nucleus had vanished. In general, these meteors appeared to move
either in straight or slightly curved orbits ; but quite a number
among them exhibited very extraordinary motions, and followed
very complicated paths, some of which were quite incomprehensible.

While some moved either in wavy or zig-zag lines, strongly
accentuated, others, after moving for a time in a straight line, grad-
ually changed their course, curving upward or downward, thus
moving in a new direction. Several among them, which were ap-
parently moving in a straight line with great rapidity, suddenly
altered their course, starting at an abrupt angle in another direc-
tion, with no apparent slackening in their motion. One of them,
which was a very conspicuous object, was moving slowly in a
straight course, when of a sudden it made a sharp turn and con-
tinued to travel in a straight line, at an acute angle with the first,
retreating, and almost going back towards the regions from which
it originally came. As nearly all the meteors which exhibited these
extraordinary motions left the trace of their passage in the sky by a
luminous trail, it was easily ascertained that these appearances were
not deceptive. On one occasion I noticed that the change of direc-
tion in the orbit corresponded with the brightening up of the meteor
thus disturbed in its progress.

Among these meteors, some traveled very slowly, and a few
seemed to advance as if by jerks, but in general they moved very
rapidly. One of the meteors thus appearing to move by jerks left a
luminous trail, upon which the various jerks seemed to be left im-
pressed by a succession of bright and faint spaces along the train.
Some of the largest meteors appeared to rotate upon an axis as
they advanced, and most of these revolving meteors, as also a great
number of the others, seemed to explode just before they disap-
peared, sending bright fiery sparks of different colors in all direc-
tions, although no. sound was at any time heard. The largest and
most brilliant meteor observed on that night appeared at 5h. 3Om.,
a little before sunrise. It was very bright, and appeared consider-
ably larger than Venus, having quite a distinct disk. This meteor
moved very slowly, leaving behind a large phosphorescent trail,
which seemed to issue from the inside of the nucleus as it advanced.
For a moment the train increased in size and brightness close to
the nucleus, which then appeared as an empty transparent sphere,

118 THE TROUVELOT

sprinkled all over with minute fiery sparks ; the nucleus then sud-
denly burst out into luminous particles, which immediately van-
ished, only the luminous trail of considerable dimensions being left.

Many of the trails thus left by the meteors retained their lumin-
osity for several minutes, and sometimes for over a quarter of an
hour. These trails slowly changed their form and position; but it
is perhaps remarkable that almost all those which I observed on
that night assumed the same general form — that of an open, irregu-
lar ring, or horse-shoe, somewhat resembling the letter C. This
ring form was subsequently transformed into an irregular, roundish
cumulus-like cloud. The trail left by a very large meteor, which
I observed on the evening of September 5th, 1880, also exhibited
the same general character of transformation.

While I was observing a long brilliant trail left by a meteor
on the night of November I3th, 1868, it was suddenly crossed by
another bright shooting-star. The latter apparently went through
the luminous substance forming the trail, which was suddenly altered
in form, and considerably diminished in brightness simultaneously
with this passage, although electrical action at some distance might
perhaps as well explain the sudden change observed.

In the majority of cases the meteors appeared white; but many,
especially the largest, exhibited a variety of brilliant colors, among
which the red, blue, green, yellow and purple were the most com-
mon. In general the trails exhibited about the same color as the
nucleus, but much fainter, and they were usually pervaded by a
greenish tint. In some instances the trails were of quite a different
color from the nucleus.

The luminous cloud observed at 5h. 3Om. on the morning of No-
vember I4th, 1868, after having passed through the series of trans-
formations above described, remained visible for a long while after
sunrise, appearing then as a small cirrus cloud, exactly similar in
appearance to the hundreds of small cirrus clouds then visible in the
sky, which had probably the same meteoric origin. For over three
hours after sunrise, these cirrus clouds remained visible in the sky,
moving all together with the wind in the high regions of the atmo-
sphere.

Although Plate XII. is intended to represent all the character-
istics exhibited by the meteors observed on that night, every form
represented having been obtained by direct observation, yet the
number is much greater than it was at any single moment during
the particular shower of 1868. As regards number, the intention
was to give an idea of a great meteoric shower, such as that of 1833,

ASTRONOMICAL DRA WINGS. 119

for instance. Although many of the falling stars seem to be close
to the Earth's surface, yet this is only an effect of perspective due to
their great distance, very few of these meteors ever coming into the
lower regions of our atmosphere at all.

The phenomena exhibited during other great meteoric showers
have been similar to those presented by the shower just described,
the only differences consisting in variations of size and brightness
in the meteors, and also in the trails, which sometimes are not so
numerous as they were in 1868.

While some shooting-stars move so rapidly that they can hardly
be followed in their orbits, others move so slowly that the sight can
easily follow them, and even remark the peculiarities of their move-
ments, some remaining visible for half a minute. Some of the fall-
ing stars move at the rapid rate of 100 miles a second, but others
only 10 miles a second, and even less. In general, they move about
half as fast again as the Earth in its orbit. The arcs described by
the meteors in the sky are variable. While some extend 80° and
even 100°, others are hardly half a degree in length. While some
shooting-stars are so faint that they can hardly be seen through the
largest telescopes, others are so large and brilliant that they can be
seen in the day-time. In general, a shooting-star of average bright-
ness resembles a star of the third or fourth magnitude.

Whatever may be the origin of the shooting-stars, they are, when
we see them, not in the celestial spaces, like the planets, the comets,
or the stars, but in our atmosphere, through which they travel as
long as they remain visible. The height at which they appear and
disappear is variable, but in general they are about 80 miles above
the surface of our globe when they are first seen, and at about 55
miles when they disappear. In many cases, however, they have
been observed at greater elevations, as also at smaller. A meteor
simultaneously observed at two different stations first appeared at
the height of 285 miles, and was last seen at 192 miles above the
Earth's surface; but in rare cases the falling stars have been seen
below a layer of clouds completely covering the sky. I myself saw
one such shooting-star a few years since. The fact that the meteors
are visible at so great elevations, proves that our atmosphere ex-
tends much farther than was formerly supposed, although at these
great heights it must be extremely rarefied, and very different from
what it is in its lower regions.

There is a remarkable difference between the sporadic meteors
seen in the sky on every night, and the meteoric showers observed
only at comparatively rare intervals. While the first appear from

120 THE TROUVELOT

different points in the sky and travel in all directions, being per-
fectly independent, the meteors of a shower all come from the same
point of the heavens, from which they apparently diverge in all di-
rections. This point of divergence of the meteors is called the
radiant point of the shower. Although the meteors seem to di-
verge in all directions from the radiant point, yet they all move in
approximately parallel lines, the divergence being an effect of per-
spective.

Whatever may be the position of the radiant point in the con-
stellations, it remains as fixed in the sky as the stars themselves,
and participates with them in the apparent motion which they un-
dergo by the effect of the diurnal motion, and thus rises and sets
with the constellation to which it belongs. This fact is sufficient
to prove that the orbits of these meteors are independent of the
Earth's motion, and that consequently they do not originate in our
atmosphere. It has been shown by Encke that the radiant point
of the meteoric shower of November I3th is precisely the point
towards which our globe moves in space on November I3th; a tan-
gent to the Earth's orbit would pass through this radiant point.

The meteoric showers are particularly remarkable, not merely be-
cause of the large number of meteors which are visible and the fact that
they all follow a common orbit, but chiefly because they have a peri-
odic return, either after an interval of a year, or after a lapse of sever-
al years. At the beginning of the present century only two meteoric
showers were known, those of August loth and of November I3th,
and their periodicity had not yet been recognized, although it had
begun to be suspected. It was only in 1836 that Quetelet and Olbers
ventured to predict the reappearance of the November meteors in
the year 1867. Having made further investigations, Prof. Newton,
of Yale College, announced their return in the year 1866. In both
of these years, as also in 1868, the meteors were very numerous, and
were observed in Europe and in America on the night of November
1 3th. The predictions having thus been fulfilled, the periodicity of
the meteors was established. Since then, other periodic showers
have been recognized, although they are much less important in
regard to number than those of August and November, except that
of November 2/th, which exhibited so brilliant a display in Europe
in 1872. These successive appearances have established the main
fact that meteoric showers are more or less visible every year when
the Earth occupies certain positions in its orbit.

The meteoric shower of the loth of August has its radiant point
situated in the vicinity of the variable star Algol, in the constella-

ASTRONOMICAL DRAWINGS. 121

tion Perseus, from which its meteors have received the name of
Perseids. Although varying in splendor, this meteoric swarm never
fails to make its appearance every year. The Perseids move through
our atmosphere at the rate of 37 miles per second. The shower
usually lasts about six hours.

The meteoric shower of November I3th has its radiant point sit-
uated in the vicinity of the star Gamma, in the constellation Leo,
from which its meteors have been called Leonids. But while the
August meteors recur regularly every year, with slight variations,
the shower of November does not occur with the same regularity.
During several years it is hardly noticeable, and is even totally
absent, while in other years it is very remarkable. Every 33 years
an extraordinary meteoric shower occurs on the I3th of November,
and the phenomenon is repeated on the two succeeding years at the
same date, but with a diminution in its splendor at each successive
return. The Leonids move in an opposite direction to that of the
Earth, and travel in our atmosphere with an apparent velocity of 45
miles per second, this being about the maximum velocity observed
in falling stars. But when the motion of our globe is taken into
account, and a deduction is made of the 18 miles which it travels
per second, it is found that these meteors move at an actual mean
rate of 27 miles a second.

In a meteoric shower the stars do not fall uniformly throughout
the night, there being a time when they appear in greater numbers.
Usually it is towards morning, between 4 and 6 o'clock, that the
maximum occurs. The probable cause of this phenomenon will be
explained in its place hereafter.

The orbits of the meteoric showers are not all approximately in
the same plane, like those of the planets, but rather resemble those
of comets, and have all possible inclinations to the ecliptic. Like
the comets, too, the different meteoric showers have either direct
or retrograde motion.

The shooting-stars were formerly considered as atmospheric
meteors, caused by the combustion of inflammable gases generated
at the surface of the Earth, and transported to the high regions of
our atmosphere by their low specific gravity. But the considerable
height at which they usually appear, the great velocity of their mo-
tion, the common orbit followed by the meteors of the same shower,
and the periodicity of their recurrence, do not permit us now to en-
tertain these ideas, or to doubt their cosmical origin. But what is
their -nature ?

It is now generally admitted that innumerable minute bodies,

122 THE TROUVELOT

moving in various directions around the Sun, are scattered in the
interplanetary spaces through which our globe travels. It has been
supposed that congregations of such minute bodies form elliptical
rings, within which they are all moving in close parallel orbits around
the Sun. On the supposition that such rings intersect the orbit of
the Earth at the proper places, it was practicable to account for the
shooting-stars by the'passage through our atmosphere of the nu-
merous minute cosmical bodies composing the rings, and the Leonid
and Perseid showers were so explained. But when the elements of
the orbits of these two last swarms came to be better known, and
were compared with those of other celestial bodies, it was found
necessary to alter this theory.

It had for a long while been suspected that some kind of relation
existed between the shooting-stars and the comets. This idea,
vaguely formulated by Kepler more than two centuries ago, more
clearly expressed by Chladni, and still more by Mr. Grey, before the
British Association, at Liverpool, in 1855, has recently received a
brilliant confirmation by the researches of Professor Schiaparelli,
Director of the Observatory of Milan. A thorough investigation of
the orbits of the August and November meteors led Schiaparelli to
the discovery of a remarkable relation between meteoric and comet-
ary orbits. By comparing the elements of these meteoric orbits
with those of comets, he found a very close resemblance between the
orbit of the August meteors and that of the comet 1862, III., and
again between the orbit of the November meteors and that of Tem-
pers comet, 1866, I. These resemblances were too striking to be
the result of mere chance, and demonstrated the identity of these
cometary orbits with those of the Perseid and Leonid showers.
In accordance with these new facts, it is now admitted that the
meteoric showers result from the passage of our globe through
swarms of meteoric particles following the orbits of comets, which
intersect the orbit of the Earth.

Professor Schiaparelli has attempted to show how these meteoric
swarms were originally scattered along the orbits of comets, by sup-
posing these bodies to originate from nebulous masses, which, in
entering the sphere of attraction of the Sun, are gradually scattered
along their orbits, and finally form comets followed by long trails of
meteoric particles. •

It has been shown that in approaching the Sun the comets be-
come considerably elongated, their particles being disseminated
over immense distances by the solar repulsion. It seems probable
that, owing to its feeble attractive power, the nucleus is incompe-

ASTRONOMICAL DRAWINGS. 123

tent to recall the scattered cometary particles and retain them in
its grasp when they are relieved from the solar repulsion, so that
they remain free from the nucleus, although they continue to move
along its orbit. It is supposable that these cometary particles will
scatter more and more in course of time. Forming at first an elong-
ated meteoric cloud, they will finally spread along the whole orbit,
and thus form a ring of meteoric particles. Since our globe con-
stantly moves in its orbit and daily occupies a different position, it
follows that at any point where such a cometary orbit happens to
cross that of the Earth, our globe will necessarily encounter the
cometary particles as a shower of meteors. This encounter will take
place at a certain time of the year, either yearly, if they form a con-
tinuous ring, or after a succession of years, if they simply form an
elongated cloud. Such meteoric clouds or rings would not be visi-
ble in ordinary circumstances, even through the largest telescopes,
except on penetrating the upper regions of our atmosphere, when
they would appear as showers of falling stars. It is supposed that
in penetrating our atmosphere, even in its most rarefied regions,
these meteors are heated by the resistance offered by the air to
their motion, first becoming luminous and then being finally vapor-
ized and burnt before they can reach the surface of the Earth.

The orbit of the comet of 1862, III., which so closely corresponds
with that of the Perseid meteors, is much more extended than that
of Tempel's comet corresponding with that of the Leonids. While
the first extends far beyond the orbit of Neptune, the latter only goes
a little beyond that of Uranus. The former orbit makes a consider-
able angle with the plane of the Earth's orbit, but the latter is much
nearer to parallelism with it. The period of revolution of the first is
108 years, and that of the last about 33^ years.

From the fact that the Perseid shower occurs yearly on the loth
of August, when the Earth crosses the orbit of the comet of 1862,
III., it is supposed that the cometary particles producing this shower
are disseminated along the whole orbit, and form a ring encircling
the Sun and Earth. To explain the yearly variations in the number
of the shooting-stars observed, these particles are supposed to be un-
equally distributed over the orbit, being more crowded at one place
than they are at another. In order to explain the meteoric shower
of Leonids, which appears in all its splendor every 33 years, and
then with diminished intensity for two successive years, after which
it is without importance, it is supposed that the cometary particles
of the comet of 1866, I., have not as yet spread all along the orbit,
a sufficient time not having been allowed, but form an elongated

124 THE TROUVELOT

meteoric cloud, more dense in its front than in its rear part. From
these considerations it has been supposed also that the comet of
1866, I., is of a more recent date than that of 1862, III. While
Tempel's comet makes its revolution around the Sun in about 33
years, this meteoric cloud, which has the same period and returns to
the same point of its orbit every 33 years, encounters our globe for
three successive years. The first year we are passing through its
densest parts, and the two following years in less and less crowded
parts, from which result the observed phenomena. An idea of the
extent of this meteoric cloud may be formed from the fact that, with
its cometary velocity of motion, it takes this cloud three years at
least to cross the Earth's orbit. From recent researches it would
appear that the Leonid cloud is not single, but that at least two
others of smaller importance exist, and have periods of 33^ years.

Biela's comet, which was divided into two parts in 1846, is
another of the few comets whose orbit approaches that of the
Earth. Possessing this knowledge, and knowing then the close con-
nection existing between meteors and comets, astronomers supposed
that there were sufficient reasons to expect a meteoric shower when
this comet was passing near the Earth. They consequently expect-
ed a meteoric display in 1872, when our globe was to cross its orbit.
Their anticipation was plainly fulfilled, and on the night of Novem-
ber 27th, 1872, a splendid meteoric display, having its radiant point
in the constellation Andromeda, was observed in Europe, and also
in America, but the meteors seen here were not so numerous as in Eu-
rope. Other meteoric showers of less importance, such as that of
April 2Oth, for instance, have also been identified with cometary or-
bits, so that now no doubt seems to remain as to the identity of com-
etary particles and shooting-stars.

The fact that the maximum number of meteors is always ob-
served in the morning hours, supports the hypothesis of the cosmic
origin of the shooting-stars, since the regions of the Earth where it
is morning are precisely those fronting the regions towards which
our globe is moving in space, and accordingly encounter more di-
rectly the meteors moving in their orbit. The greater abundance
of falling stars at that time may thus be accounted for.

The number of meteors penetrating our atmosphere must be
very great; there is not an hour and probably not a minute during
which none fall. From various considerations, some astronomers
have estimated at from 65,000,000,000 to 146,000,000,000 the total
number of shooting-stars yearly penetrating in our atmosphere.
The actual number is undoubtedly great, yet the fact that the

ASTRONOMICAL DRAWINGS. 125

meteors are rarely seen through the telescope while employed in
observing various celestial objects, does not indicate that they are
so numerous as these figures imply. It is only occasionally that
one is seen traversing the field of the instrument. Even when the
sky is observed with a low power eye-piece for several hours in suc-
cession, many nights may pass without disclosing one, although
an observer, sweeping the sky more freely with the naked eye, may
often perceive four or five during an ordinary night.

About the true nature of these bodies nothing is known with
certainty. From spectrum analysis it seems to be established that
most of them contain sodium and magnesium, while a few indicate
the presence of strontium and iron, and in some rare cases there are
traces of coal-gas. Some of the nuclei give a continuous spectrum,
and others a spectrum of lines. The trail always gives a spectrum
of bright lines which indicates its gaseous state. The traces of
coal-gas rarely seen in meteors are, however, of great importance,
as it identifies them more closely with the comets, which generally
show a similar spectrum. The continuous spectra exhibited by
some nuclei would indicate that they are incandescent and either
solid or liquid; but it is difficult to conclude from their spectra what
is their true nature, since we do not know exactly what part the
terrestrial atmosphere may play in producing the results.

The mass of the shooting-stars is not known with certainty, but
the fact that during great meteoric showers, none are seen to reach
the surface of the Earth, all being consumed in a few seconds, suf-
ficiently indicates that it must be very small. It has been calculated
that those equal to Venus in apparent size and brilliancy may weigh
several pounds, while the faint ones would weigh only a few grains.

If the shooting-stars have even such a mass as that here attrib-
uted to some of them, the extraordinary motions which I have de-
scribed above seem to be unaccountable. The change of direction
of a heavy mass moving swiftly cannot be sudden. The semi-cir-
cular, the wavy and the angular orbits observed could not be de-
scribed, it would seem, by such a mass animated with a great ve-
locity. Although the meteors are said to be ignited by the trans-
formation of part of their progressive motion into molecular motion,
yet it is not observed that the velocity of the falling stars diminishes
when they are about to disappear. The luminous trails they leave
in the atmosphere do not appear to be endowed with any motion,
but remain for a time in their original positions. These facts are
apparently opposed to the hypothesis that such meteors have any
appreciable mass. The extraordinary motions exhibited by some

126 THE TROUVELOT

meteors seem to indicate that some unsuspected force resides in
these bodies, and causes them to deviate from the laws of ordinary
motion.

Although it is very probable that the ordinary shooting-stars
have no appreciable mass, yet it is known that very heavy meteoric
masses sometimes fall at the surface of the Earth. Such falls are
generally preceded by the sudden apparition in the sky of a large,
and usually very brilliant fire-ball, which traverses the air at a great
speed, sometimes leaving behind it a luminous trail, after which it
explodes with a loud sound, and heavy fiery meteoric fragments,
diverging in all directions, fall at the surface of the Earth. The
name of Aerolites or Meteor elites is given to these ponderous frag-
ments. As these meteors, before they explode and fall to the ground,
have many points of resemblance with the shooting-stars, they are
generally supposed to be connected with them, and to have a simi-
lar cometary origin. The fact that the aerolites differ widely from
each other in constitution, and are all composed of substances found
on the Earth, associated with other facts given below, would rather
seem to indicate a terrestrial than a celestial origin.

If the aerolites belong to the same class of bodies as the falling
stars, differing from them only in size and mass, it is difficult to see
why so very few should fall upon the Earth during the great meteoric
showers, when thousands of shooting-stars traverse our atmosphere.
In Prof. Kirkwood's "Meteoric Astronomy" are given catalogues
of all the falls of aerolites and fire-balls which have been observed
at the time of the periodic meteoric showers of the roth of August
and the I3th of November, during a period of 221 years for the
Perseids, or August showers, and of 318 years for the Leonids, or
November showers. During 221 years, 10 falls of aerolites have been
witnessed simultaneously with the fall of the Perseids; while during
318 years, only 4 such falls have been recorded as having occurred
at the time of the Leonid shower. If there is any close connection
between the shooting-stars and the aerolites, we should expect to
find a maximum in their fall at the time of the great meteoric .dis-
plays. So far, no maxima or minima have yet been discovered in
the fall of aerolites; they do not seem, like meteoric showers, to be
governed by a law of periodicity.

A very remarkable peculiarity of the aerolites is that they seem
to have a tendency to fall in certain regions. Such are the southern
part of France, the north of Italy, Hindostan, the central states of
North America, and Mexico and Brazil. There is a curious contrast
existing between the quick cometary motion of the aerolites before

ASTRONOMICAL DRAWINGS. 127

their explosion, and the comparatively slow motion of their frag-
ments as they reach the Earth; motion which seems to be no greater
than that corresponding to their natural fall impeded by the resist-
ance of the air. In general, their penetration into the soil upon
which they fall does not at all correspond to the great velocity with
which they move in the atmosphere. The fragmentary structures
of the aerolites, their identity of substance with that of our globe,
their great resemblance to the volcanic minerals of the Earth, and
the fractures and faults which some of them exhibit, do not corre-
spond at all with the idea that they are cometary particles fallen on
the Earth. As far as their structure and appearance is concerned,
they seem rather to be a volcanic product of the interior of the
Earth than parts of disintegrated comets. It must be admitted that
their identity with the shooting-stars is far from established, and
that they are still involved in mystery.

The so-called meteoric dust gathered at sea and on high mount-
ains may have various origins, and may be partly furnished by vol-
canic dust carried to great distances in the atmosphere.

Since millions of shooting-stars penetrate our atmosphere every
year and remain in it, becoming definitively a part of the Earth, it
follows that, no matter how small may be the quantity of matter of
which they are composed, they must gradually increase the volume
and mass of our globe, although the increase may be exceedingly
slow. Supposing every one of the shooting-stars penetrating our
atmosphere to contain one cubic millimeter of matter, it has been
calculated that it would take nearly 35,000 years to make a deposit
one centimeter in thickness all over the surface of our globe. In-
significant as this may appear, it is probable that the quantity of
matter of meteoric origin which is added to our globe is much less
than has just been supposed.

128 THE TROUVELOT

THE MILKY-WAY OE GALAXY.
PLATE XIII.

DURING clear nights, when the Moon is below the horizon, the
starry vault is greatly adorned by an immense belt of soft white
light, spanning the heavens from one point of the horizon to the
opposite point, and girdling the celestial sphere in its delicate folds.
Every one is familiar with this remarkable celestial object, called
the Milky-way or Galaxy.

Seen with the naked eye, the Galaxy appears as an irregular,
narrow, nebulous belt, apparently composed of cloud-like luminous
masses of different forms and sizes, separated by comparatively dark
intervals. These cloud-like masses vary much in luminous intensity,
and while some among them are very bright and conspicuous, others
are so faint that they are hard to recognize. In general, the bright-
est parts of the Milky- way are situated along the middle of its belt,
while its borders, which are usually very faint, gradually vanish in
the sky. Some parts of the Galaxy, however, show very little of
the cloudy structure so characteristic of other parts, being almost
uniform throughout, except towards the borders, which are always
fainter. These parts showing greater uniformity are also the faintest.

Such is the general appearance of the Milky-way on ordinary
nights, but on rare occasions, when the atmosphere is particularly
pure, it presents one of the grandest sights that can be imagined.
At such favorable moments I have seen the Galaxy gleaming with
light, and appearing as if composed of star-dust or of precious stones.
The strange belt then appeared all mottled over and fleecy, its
large cloud-like masses being subdivided into numerous small,
irregular cloudlets of great brilliancy, which appeared projected upon
a soft luminous background.

The width of the Galaxy is far from being uniform; while in some
places it is only 4° or 5°, in others it is 15° and even more. In some
places it appears wavy in outline, at others quite straight; then it

ASTRONOMICAL DRAWINGS. 129

contracts, to expand a few degrees distant ; while at other places
it sends off branches and loops, varying in form, size and direction,
some of which are quite prominent, while others are very faint.

Although very irregular in form, the general appearance of the
galactic belt is that of a regular curve occupying one of the great cir-
cles of the celestial sphere. The Milky-way completely encircles
the heavens, but, of course, only one-half is visible at any one
moment, since our globe prevents the, other half from being seen.
If, for a moment, we imagine ourselves left in space, our globe hav-
ing vanished from under our feet, we should then see the whole
Galaxy forming a continuous belt in the heavens, at the centre of
which we should apparently be situated.

While only one-half of the galactic belt can be seen at once
from any point on the Earth, yet, according to the position of the
observer, a larger or smaller portion of the whole can be seen at
different times. In high northern or southern latitudes but little
more than half can be seen even by continuous observations; but as
we approach the equatorial regions, more and more of it becomes
visible, until the whole may be seen at different hours and seasons.
In the latitudes of the northern states, about two-thirds of the
Galaxy is visible, the rest remaining hidden below the horizon ;
but from the southern states very nearly the whole can be seen.
The half of the Milky-way visible at any one time from any lati-
tude on the Earth never entirely sets below the horizon, although
in some places it may be so near the horizon as to be rendered in-
visible by vapors. In the latitude of Cambridge, when in its lowest
position, the summit of its arc is still about 12° or 15° above the
northern horizon. The great circle of the celestial sphere, occupied
by the galactic belt, is inclined at an angle of about 63° to the celes-
tial equator, and intersects this great circle on one side in the con-
stellation Monoceros in 6h. 47m., and on the opposite side in the
constellations Aquila and Ophiuchus in i8h. 4/m. of right ascension;
so that its northern pole is situated in the constellation Coma Be-
renices in R. A. I2h. 47m., declination N. 27°, and the southern in
the constellation Cetus in R. A. o h. 47m., declination S. 27.°

According to the seasons and to the hours of the night at which
it is observed, the galactic arch presents different inclinations in the
sky. Owing to its inclination to the equator of the celestial sphere,
its opposite parts exhibit opposite inclinations when they pass the
meridian of a place. That part of the Galaxy which is represented
on I^late XIII., and which intersects the celestial equator in the con-
stellation Aquila, is inclined to the left or towards the east, when

130 THE TROUVELOT

it is on the meridian; while the opposite part, situated in Monocerosr
is inclined to the right, or towards the west, when it reaches the
meridian. The former passes the meridian in the evening in the
summer and autumn months; the latter, in the winter and spring
months.

By beginning at its northernmost part, represented at the upper
part of Plate XIII. , situated in "the chair" of the constellation
Cassiopeia, and descending southwardly, and continuing in the same
direction until the whole circle is completed, the course of the Milky-
way through the constellations may be briefly described as follows:
From Cassiopeia's chair, the Galaxy, forming two streams, descends
south, passing partly through Lacerta on the left, and Cepheus on
the right; at this last point it approaches nearest to the polar star.
Then it enters Cygnus, where it becomes very complicated and
bright, and where several large cloudy masses are seen terminat-
ing its left branch, which passes to the right, near the bright star
Deneb, the leader of this constellation. Below Deneb, the Galaxy
is apparently disconnected and separated from the northern part by
a narrow, irregular dark gap. From this rupture, the Milky-way
divides into two great streams separated by an irregular, dark rift.
An immense branch extends to the right, which, after having formed
an important luminous mass between the stars f and /9, continues
its southward progress through parts of Lyra, Vulpecula, Hercules,
Aquila and Ophiuchus, where it gradually terminates a few degrees
south of the equator. The main stream on the left, after having
formed a bright mass around e Cygni, passes through Vulpecula
and then Aquila, where it crosses the equinoctial just below the
star 7], after having involved in its nebulosity the bright star Altair,
the leader of Aquila. In the southern hemisphere the Galaxy be-
comes very complicated and forms a succession of very bright,
irregular masses, the upper one being in Scutum Sobieskii, while
the others are respectively situated in Sagittarius and in Scorpio;
the last, just a little above our horizon, being always considerably
dimmed by vapors. From Scutum Sobieskii, the Galaxy expands
considerably on the right, and sends a branch into Scorpio, in which
the fiery red star Antares is somewhat involved.

Continuing its course below our horizon, the Milky-way enters
Ara and Norma, and then, passing partly through Circinus, Centaurus
and Musca, it reaches the Southern Cross, after having been divided
by the large dark pear-shaped spot known to navigators as the
" Coal-Sack." In Ara and Crux the Milky-way attains its maxi-
mum of brightness, which there surpasses its brightest parts in Cyg-

ASTRONOMICAL DRA WINGS. 131

nus. In Musca, it makes its nearest approach to the south pole
of the heavens. It then enters Carina and Vela, where it spreads
out like a fan, and terminates in this last constellation, before reach-
ing /, being once more interrupted by a dark and very irregular
gap, on a line with the two star sj- and X. It is noteworthy that this
second rupture of continuity of the Galaxy in Vela is very nearly
opposite, or at about 180° from the break near Deneb in Cygnus.

Continuing its course on the other side of the break, the Milky-
way again spreads out into the shape of a fan, grows narrower in
entering Puppis, where it is longitudinally divided by darkish chan-
nels. It then passes above our southern horizon, becoming visible
to us, passing through part of Canis Major, where its border just
grazes the brilliant star Sirius. But from Puppis it gradually dimin-
ishes in brightness and complication, becoming faint and uniform.
It enters Monoceros and Orion, where it again crosses the equator a
little above d, the northernmost of the three bright stars in the
belt of Orion. Continuing its northward course it passes through
Gemini, extending as far as Castor and Pollux, and then entering
Auriga, where it begins to increase in brightness and in complica-
tion of structure. It passes partly through Camelopardus and into
Perseus, where an important branch proceeds from its southern bor-
der.

This branch beginning near the star 6r advances towards the
celebrated variable star Algol, around which it is quite bright and
complicated. Continuing its course in the same direction, the branch
rapidly loses its brightness, becoming very faint a little below Algol,
and passing through f Persei, it enters Taurus, leaving the Pleiades
on its extreme southern margin; and after having passed through ef
where it branches off, it rapidly curves towards the main stream,
which it joins near £ Tauri, thus forming an immense loop. The
ramification projecting near e Tauri involves in its nebulosity the
ruddy star Aldebaran and the scattered group of the Hyades. It
then advances towards the three bright stars d, £ and f of the belt
of Orion, which, together with the sextuple star 6 Orionis, are in-
volved in its faint nebulosity, and joins the main stream on the
equinoctial, having thus formed a second loop, whose interior part
is comparatively free from nebulosity, and contains the fine stars
Betelgeuse and Bellatrix.

That portion of the main galactic stream which is comprised be-
tween the star Deneb in Cygnus, and Capella in Auriga, is divided
longitudinally by a very irregular, narrow, darkish cleft, compara-
tively devoid of nebulosity, which, however, is interrupted at some

132 THE TROUVELOT

points. This dark gap sends short branches north and south, the
most important of which are situated near f Cephei and /9 Cas-
siopeiae. Another branch runs from 7- beyond e of the constellation
last mentioned. The main stream of the Galaxy after leaving Per-
seus, enters Cassiopeia, and sending short branches into Andromeda,
it completes its immense circle in Cassiopeia's chair, where this de-
scription was begun.

When examined through the telescope, the appearance of the
Milky- way completely changes, and its nebulous light is resolved into
an immense number of stars, too faint to be individually seen with
the naked eye. When Galileo first directed the telescope to the
galactic belt, its nebulous, cloud-like masses were at once resolved
into stars, even by the feeble magnifying power of his instrument.
When, much later, Sir William Herschel undertook his celebrated
star-gaugings of the Galaxy, millions of stars blazed out in his pow-
erful telescopes. The stars composing this great nebulous belt are
so numerous that it is impossible to arrive at any definite idea as to
their number. From his soundings Herschel estimated at 116,000
the number of stars which, on one occasion, passed through the field
of his telescope in 15 minutes, by the simple effect of the diurnal
motion of the heavens ; and on another occasion, a number estimated
at 250,000 crossed the field in 41 minutes. In a space of 5°, com-
prised between ft and 7- Cygni, shown on Plate XIII., he found no
less than 331,000 stars. Prof. Struve has estimated at 20,500,000 the
number of stars seen in the Milky- way through the twenty-foot teles-
cope employed by Herschel in his star-gaugings. Great as this num-
ber may seem, it is yet far below the truth; as the great modern
telescopes, according to Professor Newcomb, would very probably
double the number of stars seen through Herschel's largest teles-
cope, and detect from thirty to fifty millions of stars in the Milky-
way.

Although the telescope resolves the Galaxy into millions of
stars, yet the largest instruments fail to penetrate its immense depths.
The forty-foot telescope of Herschel, and even the giant telescope
of Lord Rosse, have failed to resolve the Milky-way entirely into
stars, the most distant ones appearing in them as nebulosities upon
which the nearer stars are seen projected, the galactic stratum
being unfathomable by the largest telescopes yet made.

The stars composing the Milky-way are very unevenly distrib-
uted, as might easily be supposed from the cloud-like appearance
of this belt. In some regions they are loosely scattered, forming
long rows or streams of various figures, while in others they congre-

ASTRONOMICAL DRA WINGS. 133

gate into star groups and clusters having all imaginable forms,
some being compressed into very dense globular masses. The in-
tervals left between the clustering masses are poorer in stars, and
indeed some of them are even totally devoid of stars or nebulosity.
Such are the great and small " coal-sacks " in the southern Galaxy.
I have myself detected such a dark space devoid of stars and nebu-
losity in one of the brightest parts of the Milky-way, in the con-
stellation Sagittarius, in about i/h. 45m. right ascension, and 27°
35' south declination. It is a small miniature coal-sack or opening
in the Galaxy, through which the sight penetrates beyond this great
assemblage of stars. Close to this, is another narrow opening near
a small, loose cluster.

Although lacking the optical resources which now enable us to
recognize the structure of the Milky-way, some of the ancient phi-
losophers had succeeded tolerably well in their speculations re-
garding its nature. It was the opinion of Democritus, Pythagoras
and Manilius, that the Galaxy was nothing else but a vast and con-
fused assemblage of stars, whose faint light was the true cause of
its milky appearance.

Before the invention of the telescope, no well-founded theory in
regard to the structure of the Milky-way could, of course, be at-
tempted. Although Kepler entertained different ideas in regard to
the structure of this great belt from those now generally admitted,
yet in them may be found the starting point of the modern concep-
tion of the structure of the Galaxy and of the visible universe. In
the view of this great mind, the Milky-way, with all its stars, formed
a vast system, the centre of which, and of the universe, was occupied
by our Sun. Kepler reasoned that the place of the Sun must be near
the centre of the galactic belt, from the fact this last object appears
very nearly as a great circle of the celestial sphere, and that its
luminous intensity is about the same in all its parts.

Half a century later, another attempt to explain the Milky-way
was made by Wright, of Durham, who rejected the idea of an acci-
dental and confused distribution of the stars as inconsistent with the
appearance of the Galaxy, and regarded them as arranged along a
fundamental plane corresponding to that of the Milky-way. These
ideas which were subsequently developed and enlarged by Kant,
and then by Lambert, constitute what is now known as Kant's
theory. According to this theory, the stars composing the Galaxy
are conceived as being uniformly arranged between two flat planes
of considerable extension, but which are comparatively near to-
gether, the $un occupying a place not very far from the centre of

134 THE TROUVELOT

this immense starry stratum. As we view this system crosswise
through its thinnest parts, the stars composing it appear scattered
and comparatively few in number, but when we view it lengthwise,
through its most extended parts, they appear condensed and ex-
tremely numerous, thus giving the impression of a luminous belt
encircling the heavens. In the conception of Kant, each star was a
sun, forming the centre of a planetary system. These systems are not
independent, but are kept together by the bonds of universal gravi-
tation. The Galaxy itself is one of these great systems, its princi-
pal plane being the equivalent of the zodiac in our planetary system,
while a preponderant body, which might be Sirius, is the equivalent
of our Sun, and keeps the galactic system together. In the universe
there are other galaxies, but as they are too distant to be resolved
into stars, they appear as elliptical nebulae. Such are, in brief, the
grand speculations of Kant and Lambert on the Milky-way, and the
structure of the universe.

Kant's theory rested more on conjectures than on observed
facts, and needed therefore the sanction of direct observations to
be established on a firm basis. With this view, Sir William Her-
schel investigated the subject, by a long and laborious series of ob-
servations. His plan, which was that of " star-gauging," consisted
in counting all the stars visible in his twenty-foot telescope, com-
prised in a wide belt cutting the Galaxy at right angles, and ex-
tending from one of its sides to the opposite one, thus embracing
180° of the celestial sphere. In this belt he executed 3,400 tele-
scopic star-gaugings of a quarter of a degree each, from which he
obtained 683 mean gaugings giving the stellar density of the cor-
responding regions.

The general result derived from this immense labor was that
the stars are fewest in regions the most distant from the galactic
belt; while from these regions, which correspond to the pole of the
Galaxy, they gradually increase in number in approaching the
Milky-way. The star density was found to be extremely variable,
and while some of the telescopic gaugings detected either no star at
all, or only one or two, other gaugings gave 500 stars and even more.
The average number of stars in a field of view of his telescope,,
obtained for the six zones, each of 15°, into which Herschel divided
up the portion of his observing belt, extending from the Galaxy
to its pole, is as follows: In the first zone, commencing at 90^
from the galactic belt and extending towards it, 4 stars per tele-
scopic field were found; 5 in the second; 8 in the third; 14 in the
fourth; 24 in the fifth and 53 in the sixth, which terminated in the

ASTRONOMICAL DRAWINGS. 135

Galaxy itself. Very nearly similar results were afterwards found by
Sir John Herschel, for corresponding regions in the southern hemi-
sphere.

From these studies, Herschel concluded that the stellar sys-
tem is of the general form supposed by the Kantian theory, and
that its diameter must be five times as extended in the direction of
the galactic plane, as it is in a direction perpendicular to it. To
explain the great branch sent out by the Galaxy in Cygnus, he sup-
posed a great cleft dividing the system edgewise, about half way
from its circumference to its centre. From suppositions founded on
the apparent magnitude and arrangement of stars, he estimated that
it would take light about 7,000 years to reach us from the extremi-
ties of the Galaxy, and therefore 14,000 years to travel across the
system, from one border to the opposite one.

But Herschel's theory concerning the Milky-way rested on the
erroneous assumption that the stars are uniformly distributed in
space, and also that his telescopes penetrated through the entire
depth of the Galaxy. Further study showed him that his telescope
of twenty feet, and even his great forty-foot telescope, which was
estimated to penetrate to a distance 2,300 times that of stars of the
first magnitude, failed to resolve some parts of the Galaxy into
stars. Meanwhile, the structure of the Milky-way being better
known, the irregular condensation of its stars became apparent,
while the mutual relation existing between binary and multiple sys-
tems of stars, as also between the stars which form clusters, was
recognized, as showing evidence of closer association between cer-
tain groups of stars than between the stars in general. Herschel's
system, which rested on the assumption of the uniform distribution
of the stars in space, and on the supposition that the telescopes used
for his gauges penetrated through the greater depths of the Galaxy,
being thus found to contradict the facts, was gradually abandoned
by its author, who adopted another method of estimating the rela-
tive distances of the stars observed in his gaugings.

This method, founded on photometric principles, consisted in
judging the penetrating power of his telescope by the brightness of
the stars, and not, as formerly, by the number which they brought
into view. He then studied by this new method the structure of the
Milky-way and the probable distance of the clustering masses of
which it is formed, concluding that the portion of the Galaxy travers-
ing the constellation Orion is the nearest to us. This last result
seems indicated by the fact that this portion of the Milky-way is the
faintest and the most uniform of all the galactic belt.

136

THE TROUVELOT

More recently Otto Struve investigated the same subject, and
arrived at very nearly similar conclusions, which may be briefly stated
as follows : The galactic system is composed of a countless num-
ber of stars, spreading out on all sides along a very extended
plane. These stars, which are very unevenly distributed, show a
decided tendency to cluster together into individual groups of differ-
ent sizes and forms, separated by comparatively vacant spaces.
This layer where the stars congregate in such vast numbers may
be conceived as a very irregular flat disk, sending many branches
in various directions, and having a diameter eight or ten times its
thickness. The size of this starry disk cannot be determined, since
it is unfathomable in some directions, even when examined with
the largest telescopes. The Sun, with its attending planets, is in-
volved in this immense congregation of suns, of which it forms but
a small particle, occupying a position at some distance from the
principal plane of the Galaxy. According to Struve, this distance
is approximately equal to 208,000 times the radius of the Earth's
orbit. The Milky-way is mainly composed of star-clusters, two-
thirds, perhaps, of the whole number visible in the heavens being in-
volved in this great belt. In conclusion, our Sun is only one of the
individual stars which constitute the galactic system, and each of
these stars itself is a sun similar to our Sun. These individual suns
are not independent, but are associated in groups varying in num-
ber from a few to several thousands, the Galaxy itself being noth-
ing but an immense aggregation of such clusters, whose whole
number of individual suns probably ranges between thirty and fifty
millions. In this vast system our globe is so insignificant that it can-
not even be regarded as one of its members. According to Dr.
Gould, there are reasons to believe that our Sun is a member of
a small, flattened, bifid cluster, composed of more than 400 stars,
ranging between the first and seventh magnitude, its position in
this small system being eccentric, but not very far from the galactic
plane.

The study of the Milky-way, of which Plate XIII. is only a part,
was undertaken to answer a friendly appeal made by Mr. A. Marth,
in the Monthly Notices of the Royal Astronomical Society, in 1872.
I take pleasure in offering him my thanks for the suggestion, and for
the facility afforded me in this study by his " List of Co-ordinates of
Stars within and near the Milky-way," which was published with it.

ASTRONOMICAL DRAWINGS. 137

THE STAR-CLUSTERS.
PLATE XIV.

IT is a well-known fact that the stars visible to the naked eye
are very unequally distributed in the heavens, and that while they
are loosely scattered in some regions, in others they are compara-
tively numerous, sometimes forming groups in which they appear
quite close together.

In our northern sky are found a few such agglomerations of stars,
which are familiar objects to all observers of celestial objects. In
the constellation Coma Berenices, the stars are small, but quite
condensed, and form a loosely scattered, faint group. In Taurus,
the Hyades and the Pleiades, visible during our winter nights, are
conspicuous and familiar objects which cannot fail to be recognized.
In the last group, six stars may be easily detected by ordinary eyes
on any clear night, but more can sometimes be seen; on rare occa-
sions, when the sky was especially favorable, I have detected eleven
clearly and suspected several others. The six stars ordinarily visi-
ble, are in order of decreasing brightness, as follows : Alcyone,
Electra, Atlas, Maia, Taygeta and Merope. Glimpses of Celano and
Pleione are sometimes obtained.

When the sky is examined with some attention on any clear,
moonless night, small, hazy, luminous patches, having a cometary
aspect, are visible here and there to the naked eye. In the constel-
lation Cancer is found one of the most conspicuous, called Praesepe,
which forms a small triangle with the two stars 7- and d. In Perseus,
and involved in the Milky-way, is found another luminous cloud,
situated in the sword-handle, and almost in a line with the two
stars f and d of Cassiopeia's Chair. In the constellation Hercules,
another nebulous mass of light, but fainter, is also visible between
the stars y and £, where it appears as a faint comet, in the depths
of space. In Ophiuchus and Monoceros are likewise found hazy, lu-
minous patches. In the southern sky, several such objects are also
visible to the naked eye, being found in Sagittarius, in Canis Major

138 THE TROUVELOT

and in Puppis; but the most conspicuous are those in Centaurus and
Toucan. That in Centaurus involves the star to in its pale diffused
nebulosity, and that in Toucan is involved in the lesser Magellanic
cloud.

When the telescope is directed to these nebulous objects, their
hazy, ill-defined aspect disappears, and they are found to consist of
individual stars of different magnitudes, which being more or less
closely grouped together, apparently form a system of their own.
These groups, which are so well adapted to give us an insight into
the structure and the vastness of the stellar universe, are called
Star-clusters.

Star-clusters are found of all degrees of aggregation, and while
in some of them, such as in the Pleiades, in Praesepe and in Perseus,
the stars are so loosely scattered that an opera glass, and even the
naked eye, will resolve them; in others, such as in those situated in
Hercules, Aquarius, Toucan and Centaurus, they are so greatly
compressed that even in the largest telescopes they appear as a
confused mass of blazing dust, in which comparatively few individual
stars can be distinctly recognized. Although only about a dozen
Star-clusters can be seen in the sky with the naked eye, yet nearly
eleven hundred such objects visible through the telescope, have
been catalogued by astronomers.

The stars composing the different clusters visible in the heavens
vary greatly in number, and while in some clusters there are only a
few, in others they are so numerous and crowded that it would be
idle to try to count them, their number amounting to several thou-
sands. It has been calculated by Herschel that some clusters are
so closely condensed, that in an area not more than TV part of that
covered by the Moon, at least 5,000 stars are agglomerated.

When the group in the Pleiades is seen through the telescope it
appears more important than it does to the naked eye, and several
hundreds of stars are found in it. In a study of Tempel's nebula,
which is involved in the Pleiades, I have mapped out 250 stars,
mostly comprised within this nebula, with the telescope of 6j^ inch-
es aperture, which I have used for this study.

As a type of a loose, coarse cluster, that in Perseus is one of the
finest of its class. It appears to the naked eye as a single object,
but in the telescope it has two centres of condensation, around
which cluster a great number of bright stars, forming various curves
and festoons of great beauty. Among its components are found
several yellow and red stars, which give a most beautiful contrast
of colors in this gorgeous and sparkling region. In a study which I

ASTRONOMICAL DRAWINGS. 139

have made of this twin cluster, I have mapped out 664 stars be-
longing to it, among which are two yellow and five red stars.

While some clusters, like those just described, are very easily
resolvable into stars with the smallest instruments, others yield with
the greatest difficulty, even to the largest telescopes, in which their
starry nature is barely suspected. Owing to this peculiarity, star-
clusters are usually divided into two principal classes. In the first
class are comprised all the clusters which have been plainly resolved
into stars, and in the second all those which, although not plainly
resolvable with the largest instruments now at our disposal, show a
decided tendency to resolvability, and convey the impression that
an increase of power in telescopes is the only thing needed to re-
solve them into stars. Of course this classification, which depends
on the power of telescopes to decide the nature of these objects, is
arbitrary, and a classification based on spectrum analysis is now sub-
stituted for it.

The star-clusters are also divided into globular and irregular
clusters, according to their general form and appearance. The
globular clusters, which are the most numerous, are usually well-
defined objects, more or less circular in their general outlines. The
rapid increase of brightness towards their centres, where the stars
composing them are greatly condensed, readily conveys the im-
pression that the general form of these sparkling masses is globular.
The irregular clusters are not so rich in stars as the former. Usually
their stars are less condensed towards the centre, and are, for the
most part, so loosely and irregularly distributed, that it is impossi-
ble to recognize the outlines of these clusters or to decide where
they terminate. The globular clusters are usually quite easily re-
solvable into stars, either partly or wholly, although some among
them do not show the least traces of resolvability, even in the
largest instruments. This may result from different causes, and
may be attributed either to the minuteness of their components or
to their great distance from the Earth, many star-clusters being at
such immense distances that they are beyond our means of measure-
ment.

As has been shown in the preceding section, the star-clusters
are found in great number in the Galaxy; indeed, it is in this region
and in its vicinity that the greater portion of them are found. In
other regions, with the exception of the Magellanic clouds, where
they are found in great number and in every stage of resolution, the
clusters are few and scattered.

The star-cluster in the constellation Hercules, designated as No.

140

THE TROUVELOT

4,230 in Sir J. Herschel's catalogue, and which is represented on
Plate XIV., is one of the brightest and most condensed in the
northern hemisphere, although it is not so extended as several
others, its angular diameter being only 7' or 8'. This object, which
was discovered by Halley in 1714, is one of the most beautiful of its
class in the heavens. According to Herschel, it is composed of
thousands of stars between the tenth and fifteenth magnitudes.
Undoubtedly the stars composing this group are very numerous, al-
though those which can be distinctly seen as individual stars, and
whose position can be determined, are not so many as a superficial
look at the object would lead us to suppose. From a long study of
this cluster, which I have made with instruments of various aper-
tures, I have not been able to identify more stars than are repre-
sented on the plate, although the nebulosity of which this object
mainly consists, and especially the region situated towards its cen-
tre, appeared at times granular and blazing with countless points
of light, too faint and too flickering to be individually recognized.
Towards its centre there is quite an extended region, whose lumin-
ous intensity is very great, and which irresistibly conveys the im-
pression of the globular structure of this cluster. Besides several
outlying appendages, formed by its nebulosity, the larger stars re-
cognized in this cluster are scattered and distributed in such a way
that they form various branches, corresponding with those formed by
the irresolvable nebulosity. At least six or seven of these branches
and wings are recognized, some of which are curved and bent in
various ways, thus giving this object a distant resemblance to
some crustacean forms. Although I have looked for it with care, I
have failed to recognize the spiral structure attributed to this object
by several observers. Among the six appendages which I have
recognized, some are slightly curved; but their curves are sometimes
in opposite directions, and two branches of the upper portion make
so short a bend that they resemble a claw rather than a spiral
wing. The spectrum of this cluster, like that of many objects of its
class, is continuous, with the red end deficient.

A little to the north-east of this object is found the cluster No.
4,294, which, although smaller and less bright than the preceding, is
still quite interesting. It appears as a distinctly globular cluster
without wings, and much condensed towards its centre. The stars
individually recognized in it, although less bright than those of the
other cluster, are so very curiously distributed in curved lines that
they give a peculiar appearance to this condensed region.

A little to the north of 7- Centauri may be found the great

ASTRONOMICAL DRAWINGS. 141

to Centauri cluster, No. 3,531, already referred to above. This
magnificent object, which appears as a blazing globe 20' in diame-
ter, is, according to Herschel, the richest in the sky, and is resolved
into a countless number of stars from the twelfth to the fifteenth
magnitude, which are greatly compressed towards the centre. The
larger stars are so arranged as to form a sort of net-work, with two-
dark spaces in the middle.

The great globular cluster No. 52, involved in the lesser Magel-
lanic cloud, in the constellation Toucan, is a beautiful and remark-
able object. It is composed of three distinct, concentric layers of
stars, varying in brightness and in degree of condensation in each
layer. The central mass, which is the largest and most brilliant, is;
composed of an immense number of stars greatly compressed, whose,
reddish color gives to this blazing circle a splendid appearance.
Around the sparkling centre is a broad circle, composed of less
compressed stars, this circle being itself involved in another cir-
cular layer, where the stars are fainter and more scattered and
gradually fade away.

Many other great globular clusters are found in various parts of
the heavens, among which may be mentioned the cluster No. 4,678,.
in Aquarius. This object is composed of several thousand stars of
the fifteenth magnitude, greatly condensed towards the centre, and,
as remarked by Sir J. Herschel, since the brightness of this cluster
does not exceed that of a star of the sixth magnitude, it follows that
in this case several thousand stars of the fifteenth magnitude equal
only a star of the sixth magnitude. In the constellation Serpens
the globular clusters No. 4,083 and No. 4,1 18 are both conspicuous
objects, also No. 4,687 in Capricornus. In Scutum Sobieskii the
cluster No. 4,437 is one of the most remarkable of this region. The
stars composing it, which are quite large and easily made out separ-
ately, form various figures, in which the square predominates.

Among the loose irregular clusters, some are very remarkable for
the curious arrangement of their stars. In the constellation Gemini
the cluster No. 1,360, which is visible to the naked eye, is a magni-
ficent object seen through the telescope, in which its sparkling stars
form curves and festoons of great elegance. The cluster No. 1,467,
of the same constellation, is remarkable for its triangular form. In
the constellation Ara the cluster No. 4,233, composed of loosely
scattered stars, forming various lines and curves, is enclosed orr
three sides by nearly straight single lines of stars. In Scorpio the
cluster No. 4,224 is still more curious, being composed of a continu-
ous ring of loosely scattered stars, inside of which is a round, loose-

142 THE TROUVELOT

cluster, which is divided into four parts by a dark cross-shaped gap,
in which no stars are visible.

Among the 1,034 objects which are now classified as clusters
more or less resolvable, 565 have been absolutely resolved into
stars, and 469 have been only partly resolved, but are considered
as belonging to this class of objects. In Sir J. Herschel's catalogue
there are 102 clusters which are considered as being globular; among
them 30 have been positively resolved into stars.

The agglomeration of thousands of stars into a globular cluster
cannot be conceived, of course, to be simply the result of chance.
This globular form seems clearly to indicate the existence of some
bond of union, some general attractive force acting between the
different members of these systems, which keeps them together,
and condenses them towards the centre. Herschel regards the
loose, irregular clusters as systems in a less advanced stage of con-
densation, but gradually concentrating by their mutual attraction
into the globular form. Although the stars of some globular clus-
ters appear very close together, they are not necessarily so, and
may be separated by great intervals of space. It has been shown
that the clusters are agglomeration of suns, and that our Sun itself
is a member of a cluster composed of several hundreds of suns,
although, from our point of observation, these do not seem very close
together. So far as known, the nearest star to us is a Centauri, but
its distance from the Earth equals 221,000 times the distance of the
Sun from our globe, a distance which cannot be traversed by light
in less than three years and five months. It seems very probable
that if the suns composing the globular clusters appear so near
together, it is because, in the first place, they are at immense dis-
tances from us, and in the second, because they appear nearly in
a line with other suns, which are at a still greater distance from
us, and on which they accordingly are nearly projected. If one
should imagine himself placed at the centre of the cluster in Her-
cules, for instance, the stars, which from our Earth seems to be so
closely grouped, would then quite likely appear very loosely scat-
tered around him in the sky, and would resemble the fixed stars as
seen from our terrestrial station.

Judging by their loose and irregular distribution, the easily re-
solvable clusters would appear, in general, to be the nearer to us.
It is probable that the globular clusters do not possess, to a very
great degree, the regular form which they ordinarily present to
us. It seems rather more natural to infer that they are irregular,
and composed of many wings and branches, such as are observed

ASTRONOMICAL DRA WINGS. 143

in the cluster in Hercules; but as these appendages would neces-
sarily be much poorer in stars than the central portions, they would
be likely to become invisible at a great distance, and therefore the
object would appear more or less globular; the globular form being
simply given by the close grouping of the stars in the central por-
tion. It would seem, then, that in general, the most loosely scat-
tered and irregular clusters are the nearest to us, while the smallest
globular clusters and those resolvable with most difficulty are the
most distant.

In accordance with the theory that the clusters are composed of
stars, the spectrum of these objects is in general continuous; al-
though, in many cases, the red end of the spectrum is either very
faint or altogether wanting. Many objects presenting in a very
high degree the principal characteristics exhibited by the true star-
clusters, namely, a circular or oval mass, whose luminous intensity
is greatly condensed toward the centre, have not yielded, however,
to the resolving power of the largest telescopes, although their con-
tinuous spectrum is in close agreement with their general resem-
blance to the star-clusters. Although such objects may remain
irresolvable forever, yet it is highly probable that they do not mate-
rially differ from the resolvable and partly resolvable clusters, except
by their enormous distance from us, which probably reaches the ex-
treme boundary of our visible universe.

144 THE TROUVELOT

THE NEBULAE.
PLATE XV.

Besides the foggy, luminous patches which have just been de-
scribed, a few hazy spots of a different kind are also visible to the
naked eye on any clear, moonless night. These objects mainly
differ from the former in this particular, that when viewed through
the largest telescopes in existence they are not resolved into stars,
but still retain the same cloudy appearance which they present to
the unassisted eye. On account of the misty and vaporous appear-
ance which they exhibit, these objects have been called Nebula.

Of the 26 nebulous objects visible to the naked eye in the whole
heavens, 19 belong to the class of star-clusters, and 7 to the class of
nebulae. Among the most conspicuous nebulae visible to the unas-
sisted eye, are those in the constellations Argo Navis, Andromeda
and Orion.

Besides the seven nebulae visible to the naked eye, a great num-
ber of similar objects are visible through the telescope. In Sir John
Herschel's catalogue of nebulae and clusters, are found 4,053 irre-
solvable nebulae, and with every increase of the aperture of tele-
scopes, new nebulae, invisible in smaller instruments, are found.
Notwithstanding their irresolvability it is probable, however, that
many among them have a stellar structure, which their immense
distance prevents us from recognizing, and are not therefore true
nebulae. The giant telescope of Lord Rosse has shown nebulae so
remote that it has been estimated that it takes their light 30 mil-
lion years to reach the Earth.

The nebulae are very far from being uniformly distributed in
space. In some regions they are rare, while in others they are nu-
merous and crowded together, forming many small, irregular groups,
differing in size and in richness of aggregation. The grouping of
the nebulae does not occur at random in any part of the heavens, as
might naturally be supposed, but, on the contrary, it is chiefly con-

ASTRONOMICAL DRAWINGS. 145

fined to certain regions. Outside of these regions nebulae are rare
and are separated from each other by immense intervals; so that
these isolated objects appear as if they were lost wanderers from
the great nebulous systems.

The regions where the nebulae congregate in great number are
very extensive, and in a general view there are two vast systems of
nebular agglomeration, occupying almost opposite points of the
heavens, whose centres are not very distant from the poles of
the Milky-way. In the northern hemisphere, the nebulous system
is much richer and more condensed than in the southern hemi-
sphere. The northern nebulae are principally contained in the con-
stellations Ursa Minor and Major, in Draco, Canes Venatici, Bootes,
Leo Major and Minor, Coma Berenices, and Virgo. In this region,
which occupies about y§ of the whole surface of the heavens, % of
the known nebulae are assembled. The southern nebulae are more
evenly distributed and less numerous, with the exception of two
comparatively small, but very remarkable centres of condensation
which, together with many star-clusters, constitute the Magellanic
clouds.

These two vast nebular groups are by no means regular in out-
line, and send various branches toward each other. They are sep-
arated by a wide and very irregular belt, comparatively free from
nebulae, which encircles the celestial sphere, and whose medial line
approximately coincides with that of the galactic belt. The Milky-
way, so rich in star-clusters, is very barren in nebulae; but it is a
very remarkable fact, nevertheless, that almost all the brightest,
largest, and most complicated nebulae of the heavens are situated
either within it, or in its immediate vicinity. Such are the great
nebulae in Orion and Andromeda; the nebula of £ Orionis; the Ring
nebula in Lyra; the bifurcate nebula in Cygnus; the Dumb-bell nebula
in Vulpecula; the Fan, Horse-shoe, Trifid and Winged nebulae in
Sagittarius; the great nebula around 57 Argus Navis, and the Crab
nebula in Taurus.

Aside from the discovery of some of the largest nebulae by differ-
ent observers, and their subsequent arrangement in catalogues by
Lacaille and Messier, very little had been done towards the study
of these objects before 1779, when Sir W. Herschel began to observe
them with the earnestness of purpose which was one of the distinct-
ive points of the character of this great man. He successively pub-
lished three catalogues in 1786, 1789, and 1802, in which the position
of 2,500 nebulous objects was given. This number was more than
doubled before 1864, when Sir John Herschel published his catalogue

146 THE TROUVELOT

of 5,079 nebulae and star-clusters. To this long list must be added
several hundred similar objects, since discovered by D'Arrest,
Stephan, Gould and others. But, as has been shown above, among*
the so-called nebulae are many star-clusters which do not properly
belong to the same class of objects, it being sometimes impossible
in the present state of our knowledge to know whether a nebulous
object belongs to one class or to the other.

The nebulae exhibit a great variety of forms and appearances,
and, in accordance with their most typical characters, they are
usually divided into several classes, which are : the Nebulous stars,
the Circular, or Planetary, the Elliptical, the Annular, the Spiral and
Irregular nebulae.

The so-called nebulous stars consist of a faint nebulosity, usually
circular, surrounding a bright and sharp star, which generally occu-
pies its centre. The nebulosity surrounding these stars varies in
brightness as well as in extent, and while, in general, its light gradu-
ally fades away, it sometimes terminates quite suddenly. Such
nebulosities are usually brighter and more condensed towards the
central star. The stars thus surrounded do not seem, however, to
be distinguished from others by any additional peculiarity. Some
nebulae of this kind are round, with one star in the centre ; others
are oval and have two stars, one at each of their foci. The nebulous
star, t Orionis, represented at the upper part of Plate XV., above the
great nebula, has a bright star at its centre and two smaller ones on
the side. The association of double stars with nebulae is very re-
markable, and may in some cases indicate a mutual relation between
them.

The so-called planetary nebulae derive their name from their
likeness to the planets, which they resemble in a more or less equa-
ble distribution of light and in their round or slightly oval form.
While some of them have edges comparatively sharp and well de-
fined, the outlines of others are more hazy and diffused. These
nebulae, which are frequently of a bluish tint, are comparatively rare
objects, and most of those known belong to the southern hemisphere.
When seen through large telescopes, however, they present a differ-
ent aspect, and their apparent uniformity changes. The largest of
these objects, No. 2,343 of the General Catalogue, is situated in the
Great Bear, close to the star /?. Its apparent diameter is, accord-
ing to Sir J. Herschel, 2', 40", and " its light is equable, except at the
edge, where it is a little hazy." In a study which I made of this ob-
ject in 1876, with a refractor 6% inches in aperture, I found it decid-
edly brighter on the preceding side, where the brightest part is

ASTRONOMICAL DRA WINGS. 147

crescent shaped. In Lord Rosse's telescope its disk is transformed
into a luminous ring with a fringed border, and two small star-like
condensations are found within. Another planetary nebula, near x
Andromedae, has also shown an annular structure in Rosse's telescope.

The elliptical nebula, as their name implies, are elongated, ellip-
tical objects ; but while some of them are only slightly elongated
ovals, others form ellipses whose eccentricity is so great that they
appear almost linear. In all these objects the light is more or less
condensed towards the centre ; but while in some of them the con-
densation is gradual and slight, in others it is so great and sudden
that the centre of the nebula appears as a large diffused star, some-
what resembling the nucleus of a comet. From the general appear-
ance of these objects, it is not unlikely that some of them are either
flattish, nebulous disks, like the planetary nebulae, or nebulous rings,
seen more or less sidewise. The condensation of light at their cen-
tres does not appear to be stellar, but nebulous like the rest, and it
is a remarkable fact that very few, if any, of these objects are resolv-
able into stars.

Several elliptical nebulae are remarkable for having a star at or
near each of their foci, or at each of their extremities. Such are the
elliptical nebulae in Draco, Centaurus, and Sagittarius, Nos. 4,419,
3,706 and 4,395 of the General Catalogue, the last of which is in the
vicinity of the triple star /Jt Sagitarii. Each of these nebulae has a
star at each of its foci, while No. I, in Cetus, has a star at each of
its extremities.

Among the most remarkable elliptic nebulae may be mentioned
Nos. 1,861 and 2,373 of Sir J. Herschel's catalogue, both situated in
the constellation Leo. The first is one of Lord Rosse's spiral nebu-
lae, and the last, which is a very elongated object, is formed of con-
centric oval rings, which are especially visible towards its central
part. The constellation Draco is particularly remarkable for the
number of elliptical nebulae found within its boundaries. Among
them are Nos. 3,939, 4,058, 4,064, 4,087, 4,415, etc., which are quite
remarkable objects of their class. No. 4,058, of which I have made
a study, is bright, and has a decided lenticular form with a condensa-
tion in the centre. Its following edge is better defined than the
preceding. In Lord Rosse's telescope this object exhibits a narrow,
dark, longitudinal, gap in its interior.

By far the largest and the finest object of this class is the great
nebula in Andromeda. Although this object belongs rather to the
class of irregular nebulae, yet it is generally considered as an elliptic
nebula, since its complicated structure, being less prominent, was not

148 THE TROUVELOT

recognized until 1848, when it was perceived by George P. Bond, Di-
rector of the Harvard College Observatory. This, the first nebula dis-
covered, was found in 1612 by Simon Marius. It is situated in the
constellation Andromeda, in the vicinity of the star v, and almost in
a line with the stars // and /9 of the same constellation. It is visible
to the naked eye, and appears as a faint comet-like object. It is
represented at the upper left hand corner of Plate XIII. , on the
border of the Milky-way, as it appears to the naked eye.

The nebula in Andromeda is one of the brightest in the heavens,
and is closely attended by two smaller nebulae. Perhaps it would be
rather more correct to say that it has three centres of condensation,
as the two small nebulae referred to are entirely involved in the
same faint and extensive nebulosity. Its general form is that of an
irregular oval, upwards of one degree in breadth and two and a
half degrees in length. Its brightest and most prominent part,
which alone was seen by the earlier observers, consists in a very
elongated lenticular mass, which gradually condenses towards its
centre into a blazing, star-like nucleus, surrounded by a brilliant
nebulous mass. At a little distance to the south of this cen-
tral condensation is found one of the lesser centres of condensa-
tion noted above, which is globular in appearance, with a bright,
star-like nucleus like the former. The other centre of condensa-
tion is found to the north-west of the centre of the principal
mass, and is quite elongated, with a centre of condensation to-
wards its southern extremity, but it is not so bright as the others.
Close to the western edge of the bright lenticular mass first de-
scribed, and making a very slight angle with its longer axis, are
found two narrow and nearly rectilinear dark rifts, running almost
parallel to each other, and both terminating in a slender point in
the south. These dark rifts, which are almost totally devoid of
nebulous matter, are quite rare in nebulae, and afford a good oppor-
tunity to watch the changes which this part of the nebulae may
undergo.

This nebula has never been positively resolved into stars, al-
though Prof. Geo. Bond and others have strongly suspected its
resolvability. In a study which I have made of it, with the same in-
strument employed by Bond, and also with the great Washington
telescope, I detected a decided mottled appearance in several
places, which might be attributable to a beginning of resolvability ;
but I do not consider this a conclusive indication that the nebula is
resolvable. The continuous spectrum given by this nebula, showing
that it is not in the gaseous state which its appearance seems to

ASTRONOMICAL DRAWINGS. 149

indicate, warrants the conclusion, however, that it will ultimately
be found to be resolvable. This object, being situated on the edge
of the Galaxy and involved in its diffused light, has a great number
of small stars belonging to this belt projected upon it. During my
observations I have mapped out 1,323 of these stars, none of which
seems to be in physical connection with the nebula.

Among the circular and elliptical nebulae a few exhibit a very
remarkable structure, being apparently perforated, and forming
either round, slightly oval, or elongated rings of great beauty.
These Annular nebulae are among the rarest objects in the heavens.
In Scorpio, two such nebulae are found involved in the Milky-way,
and also one in Cygnus. One of those in Scorpio has two stars in-
volved within its ring, at the extremities of its smallest interior
diameter. A very elongated nebula in the vicinity of the fine triple
star f Andromedae is also annular, and has two stars symmetrically
placed at the extremities of its greatest interior axis. Another
elongated annular nebula is also found north of rt Pegasi.

The grandest and most remarkable of the annular nebulae is
found in the constellation Lyra, about midway between the two
stars $ and f. It is slightly elliptical in form, and according to
Prof. E. S. Holden, its major axis is 77". 3 and the minor 58". From
a study and several drawings which I have made of this object, with
instruments of various apertures, I have found it decidedly brighter
towards its outer border, at the extremities of its minor axis, than
at the ends of the major axis. On very favorable occasions, some
of its brightest parts have appeared decidedly, but very faintly
mottled, and I have recognized three small centres of condensation.
Its interior, in which Professor Holden has detected a very faint star,
is quite strongly nebulous. In Lord Rosse's telescope, this nebula
is completely surrounded by wisps and appendages of all sorts of
forms, which I have failed to trace, however, both with the refractor
of the Harvard College Observatory and with that of the Naval
Observatory at Washington ; Rosse, Secchi and Chacornac, have
seen this nebula glittering as if it were a " heap of star dust," al-
though its spectrum indicates that it is gaseous.

The nebula No. 1,541, in Camelopardus, of which I have also made
a study and a drawing, is closely allied to the class of annular neb-
ulae. This object, which is quite bright, has a remarkable appear-
ance. It consists principally of somewhat more than half of an
oval ring, surrounding a bright, nebulous mass which condenses
around a star; this mass being separated from the imperfect ring by
a dark interval. Upon the bright portion of the ring, and on oppo-

150 THE TROL VELOT

site points, are found two bright stars, between which lies the star
occupying the central mass. The central mass extends at some dis-
tance outside of the ring on its open side. Several stars are involved
in this object.

The Spiral nebulae are very curious and complicated objects, but
they are visible only in the largest telescopes. Prominent above all
is the double spiral nebula No. 3,572, in Canes Venatici, which is
not far from y Ursa Majoris. In Lord Rosse's telescope, this object
presents a wonderful spiral disposition, looking somewhat like one
of the fire-works called pin-wheels, and forming long, curved wisps,
diverging from two bright centres. The spectrum of this object,
however, is not that of a gas. In the constellation Virgo, Rosse
has detected another such nebula. In Cepheus, Triangulum, and
Ursa Major, are found other spiral nebulae of smaller size. Lord
Rosse has recognized 40 spiral nebulae and suspected a similar
structure in 30 others.

The class of the Irregular nebulae, which will be now considered,
differs greatly in character from the others, and includes the largest,
the brightest and the most extraordinary nebulae in the heavens.
The nebulae of this class differ from those belonging to the other
classes by a want of symmetry in their form and in the distribution
of their light, as well as by their capricious shapes, and their very
complicated structure. Another and perhaps the principal difference
between them and the objects above described, consists in the re-
markable fact already stated, that they are not, except in rare cases,
to be found in the regions where the other nebulae abound. On the
contrary, they are found in or very near the Milky-way, precisely
where the other nebulae are the most rare. This fact, recognized by
Sir J. Herschel, led him to consider them as " outlying, very distant,
and as it were detached fragments of the great stratum of the Gal-
axy." It seems very probable that the reason why these objects dif-
fer so greatly from the other nebulae in size, brightness and complica-
tion of structure, is simply because they are much nearer to us than
are most of the others. They are perhaps nebulous members of our
Galaxy. The same remark which has been made of star-clusters may
be applied to nebulae. The nearer they are to us, the larger, the
brighter and the more complicated they will appear, while the far-
ther they are removed, the more simple and regular and round
they will appear, only their brightest and deepest parts being then
visible.

The Crab Nebula of Lord Rosse, near ; Tauri, No. 1,157, is one
of the interesting objects of this class. It has curious appendages

A STRONOMICA L DRA WINGS. 1 51

streaming off from an oval, luminous 'mass, which give it a distant
resemblance to the animal from which it derives its name. The
Bifid nebula in Cygnus, Nos. 4,400 and 4,616, is another object of
this class. It consists of a long, narrow, crooked streak, forking out
at several places, and passing through x Cygni. Observers, having
failed to recognize the connection existing between its different
centres of brightness, have made distinct nebulae of this extended
object.

The Dumb-bell nebula in Vulpecula, No. 4,532, is a bright and
curious object, with a general resemblance to the instrument from
which it derives its name. Lord Rosse's telescope has shown many
stars in it, projected on a nebulous background, and Prof. Bond
seems to have thought that it showed traces of resolvability, al-
though in the study which I made of this nebula with the same in-
strument used by the latter observer, I failed to perceive any such
traces. Dr. Huggins finds its spectrum gaseous.

The star-cluster, No. 4,400, in Scutum Sobieskii, which is de-
scribed by Sir J. Herschel as a loose cluster of at least 100 stars, I
have found to be involved in an extensive, although not very bright,
nebula, which would seem to have escaped his scrutiny. In a study
and drawing of this nebula made in 1876, its general form is that
of an open fan, with the exception that the handle is wanting, with
deeply indented branches on the preceding side, where the brightest
stars of the cluster are grouped. From its peculiar form, this object
might appropriately be called the Fan nebula.

The Omega or Horse-shoe nebula, in Sagittarius, No. 4,403, of
which I have made a study and two drawings, one with a refractor 6^
inches in aperture, and the other conjointly with Prof. Holden, with
the great telescope of the Naval Observatory, is a bright and very
complicated object. Its general appearance in small instruments,,
with low power, is that of a long, narrow pisciform mass of light,
from which proceeds on the preceding side, the great double
loop from which it derives its name. But in the great Washing-
ton refractor its structure becomes very complicated, forming vari-
ous bright nebulous masses and wisps of great extension. Prof.
Holden, who has made a careful, comparative study of the pub-
lished drawings of this object, thinks there are reasons to believe
that its western branch has moved relatively to the stars found
within its loop. The spectrum of this nebula is gaseous.

The Trifid nebula, No. 4,355, in the same constellation, is also a
very remarkable object, although it is not so bright as the last.
This nebula, which I have studied with the refractor of the Cam-

152 THE T^OUVELOT

bridge Observatory, consists of four principal masses of light, separ-
ated by a wide and irregular gap branching out in several places.
These masses, which are brighter along the dark gap, gradually
fade away externally. A group of stars, two of which are quite
bright, is found near the centre of the nebula, on the inner edge of
the following mass, and close to the principal branch of the dark-
channel. A little to the north, and apparently forming a part of
this nebula, is a globular-looking nebula, having a pale yellow star
at its centre. Prof. Holden's studies on this nebula show that the
triple star, which was centrally situated in the dark gap from 1784
to 1833, was found involved in the border of the nebulous mass fol-
lowing it, from 1839 to l877 5 the change, he thinks, is attributable
either to the proper motion of the group of stars or to that of the
nebula itself.

In the same vicinity is found the splendid and very extensive
nebula No. 4,361, in which is involved a loose, but very brilliant
star-cluster. This nebula and cluster, which I have studied and
drawn with a 6^3 inch telescope, is very complicated in structure,
and divided by a dark irregular gap into three principal masses of
light, condensing at one point around a star, and at others forming
long, bright, gently-curved branches, which give to this object a
strong resemblance to the wings of a bird when extended upwards
in the action of flying. From this peculiarity this object might ap-
propriately be called the Winged nebula. Its spectrum is that of a
gas.

The variable star rt Argus is completely surrounded by the great
nebula of the same name, No. 2,197, ^rst delineated by Sir J.
Herschel, during his residence at the Cape of Good Hope, in 1838.
This object, which covers more than f of a square degree, is divided
into three unequal masses, separated by dark oval spots, compara-
tively free from nebulosity, and is suspected to have undergone
changes since Herschel's time.

In the same field with the double star, £ Orionis, the most east-
erly of the three bright stars in the belt of Orion, is found another
irregular nebula of the Trifid type. From the drawings which I
have made of this object, it appears to be composed of three princi-
pal unequal masses, separated by a wide, irregular, dark channel,
two of the masses being quite complicated in structure, and forming
curved, nebulous streams of considerable length and breadth. This
nebula, like the next to be described, seems to be connected with
the Galaxy by the great galactic loop described in another section.

By far the most conspicuous irregular nebula visible from our

ASTRONOMICAL DRA WINGS. 153

northern States, is the great nebula in Orion, No. 1,179, repre-
sented on Plate XV. This object, visible to the naked eye, is the
brightest and the most wonderful nebula in the heavens. It is situ-
ated a little to the south of the three bright stars in the belt of
Orion, and may be readily detected surrounding the star 6, situated
between and in a line with two faint stars, the three being in a
straight line which points directly towards e, the middle star of the
three in Orion's belt. The area occupied by this nebula is about
equal to that occupied by the Moon.

In its brightest parts the nebula in Orion appears as a luminous
cloud of a pentagonal form, from which issue many luminous ap-
pendages of various shapes and lengths. This principal mass is
divided into secondary masses, separated by darkish, irregular inter-
vals: These secondary masses in their turn appear mottled and
fleecy. Towards the lower part of the pentagonal mass is found a
roundish dark space, comparatively devoid of nebulosity, in which
are involved four bright stars forming a trapezium, and several
fainter ones. The four bright stars of the trapezium constitute the
quadruple star 0 Orionis, from which the nebula has received its
name. The cloud-like pentagonal form is brightest on the north-
west of the trapezium, and is surrounded on three sides by long,
soft, curved wisps, fading insensibly into the outer nebulous mass in
which they are involved. On the east a broad, wavy wing spreads
out, and sends an important branch southward. South-east of the
trapezium are found several curious dark spaces, comparatively de-
void of nebulosity, especially those on the east, which give to this
nebula a singular character. Close to the north-eastern part of
the nebula, or rather in contact with it, is found a small, curiously-
shaped nebula, condensing around a bright star into a blazing nu-
cleus. From this centre it continues northward in a narrow diffused
stream, which spreads out in passing over the stars cl and c2; and
after having sent short branches northward, it curves back to the
south and joins the main nebula on the west of its starting point,
having thus formed a great loop which is not shown on the Plate.
The nebula also forms a loop towards the south, which is partly
shown on Plate XV., a small branch of which, passing through t
Orionis, the nebulous star shown at the top of the Plate, and ex-
tending southward, is not here represented.

On ordinary nights the nebula in Orion is a splendid object, and
inspires the observer with amazement; but this is as nothing com-
pared with the grand and magnificent sight which it presents dur-
ing the very rare moments when our atmosphere is perfectly clear

154 THE TROUVELOT

and steady. I have seen this nebula but once under these favorable
circumstances, and I was surprised by the grandeur of the scene.
Then could be detected features to be seen at no other time, and its
fleecy, floculent, cloud-like masses glittered with such intensity that
it seemed as if thousands of stars were going to blaze out the next
moment. Although I observed the nebula under such favorable
conditions, and with the fifteen-inch refractor of the Cambridge Ob-
servatory, yet I was disappointed in my expectations, and distin-
guished no new stars or points of light, and nothing more than a very
bright mass, finely divided into minute blazing cloudlets. Although
I failed to resolve this nebula into stars, yet Lord Rosse, Bond
and Secchi thought they had caught glimpses of star dust. Its
spectrum, however, proves to be mainly that of incandescent gases,
probably hydrogen and nitrogen. In the curved wisps found in this
nebula, Lord Rosse and others saw indications of a spiral structure.

Several bright stars are found scattered over this nebula, and be-
sides those forming the trapezium, there are three in a row, a little
to the south-east of that group, which are quite bright and remark-
able. Among the stars involved in this nebula, few show signs of
having a physical connection with it, although it seems probable
that the group of the trapezium is so connected. Some of these
stars are variable. The small stars represented on this Plate, as on
others of the series, are somewhat exaggerated in size, as was un-
avoidable with any process of reproduction which could be adopted.

In 1811, W. Herschel was led to suspect that some changes had
occurred in this nebula, but changes in such complicated and delicate
objects are not easily ascertained, since, for the most part, we have
for comparison with our later observations only coarse drawings
made by hands unskilled in delineation.

Although comparatively rare, double and multiple nebulae may
be found in the sky. When this occurs, their constituents most
commonly belong to the class of spherical nebulae. Sometimes the
components are separated and distinct, at other times one of them is
projected upon the other, either really or by the effect of perspect-
ive. Sometimes one is round and the other elongated. It is proba-
ble that while some of these nebulae are physically associated and
form a system, others appear to be so only because they happen to
be almost in a line with the observer. A double nebula in Draco,
Nos. 4,127 and 4,128, which I have drawn, is a fair type of those
which are separated. The first is a globular nebula, and the last an
oval one, with a star at its centre. The double nebula, Nos. 858 and
859, in Taurus, which I have also studied, is a type of the cases in

ASTRONOMICAL DRAWINGS. 155

which one nebula is partly projected on another. In this instance
both the nebulae are globular.

The nebulae in general show very little color in their light,
which is ordinarily whitish and pale. Some, however, present a
decided bluish or greenish tint. The great nebula in Orion has a
greenish cast, and we have seen that some planetary nebulae are
bluish.

It has been a question whether nebulae are changing. It has
already been stated that Prof. Holden believes there is ground to
suspect that the Trifid and Horse-shoe nebulae have undergone some
changes. A nebula near £ Tauri has been lost and found again
several times. Two other nebulae in the same constellation have
presented curious variations. One, near a star of the tenth magni-
tude, exhibited variations of brightness like those of the star itself,
and for a time disappeared. The other, near f Tauri, increased in
brightness for three months, after which it disappeared. In 1859
Tempel discovered a nebula in the Pleiades, which has shown some
fluctuations. In 1875 I made a long study of this object, and drew
it carefully a dozen times, but I was not able to see any changes in
it within the two or three months during which my observations were
continued. But on Nov. 24, 1876, it was found of a different color,
being purplish and very faint. On Dec. 23, 1880, it was found just as
bright and visible as when I drew it in 1875, an^ on Oct. 20, 1881,
it appeared faint and purplish again, as in 1876. On this last night,
and on those which followed it, it was impossible for me to trace
the nebulosity as far as in 1875. I consider this as due to a vari-
ation in the light of this object, which in 1875 was bright enough
to be well seen while the Moon after her First Quarter was within
ten or fifteen degrees from the Pleiades.

From the observations of M. Laugier, it appears that some
nebulae have a proper motion, comparable to that of stars. From
the displacement of the lines of their spectra by their motion in the
line of sight, Dr. Huggins found that no nebula observed by him has
a proper motion surpassing 25 miles per second. The Ring nebula
in Lyra appears, to move from us at the rate of 3 miles per second,
and that in Orion recedes about 17 miles per second.

The important question arises, are all the irresolvable nebulas
in the heavens to be considered as so many star-clusters, differing
only from them by the' minuteness of their components, or their
immense distance from us; or are they cosmical clouds, composed
of luminous vapors, similar to the matter composing the heads
and tails of comets ? Originally, W. Herschel, with many as-

156 THE TROUVELOT

tronomers, thought that all these objects were stellar aggregations,
too distant to be resolved into stars; but he subsequently modified
his opinion, and accepted the idea that some of them are of a gaseous
nature.

No direct proof that the nebulae are gaseous could be obtained,
however, before the spectroscope was known. The attempt to
analyze the light of the nebulas with this instrument was made in
1864, by Dr. Huggins, who directed his spectroscope to the planet-
ary nebula, No. 4,373, in Draco. Its spectrum was found to consist
of three bright, distinct lines, the brightest of which corresponded
with the strongest nitrogen line, and the feeblest with the hydro-
gen C line. Besides these lines, it gave also a very faint, continu-
ous spectrum, apparently due to a central point of condensation.
By this observation, the gaseous nature of a nebula was for the first
time demonstrated. Dr. Huggins thus analyzed 70 nebulae, of
which one-third gave a gaseous spectrum, consisting 6f several
bright lines, the brightest of which invariably corresponded with
the lines of nitrogen. The others gave a continuous spectrum,
with the red end usually deficient. These results indicate that if
some of the so-called nebulae are due to an aggregation of stars,
either too minute or too remote in space to be individually resolved,
others are in a gaseous state. Yet the faint, continuous spectrum,
given by some nebulae, in addition to their gaseous spectra, seems
to show that these nebulae have some stars or matter in a different
state, either involved in them or projected on their surface.

The idea of diffused matter distributed here and there in space,
and gradually condensing into stars, is by no means new. As early
as 1572, Tycho Brahe proposed such an hypothesis, to explain the
sudden apparition of a new star in Cassiopeia, which he considered
as formed by the recent agglomeration of the " celestial matter "
diffused in space. Kepler adopted the same idea to explain the
new star which appeared in Ophiuchus, in 1604. Halley, Lacaille,
Mairan and others, entertained the same opinion. The hypothesis
of a self-luminous, nebulous matter diffused in space, and forming
here and there immense masses, has been proposed from the origin
of the telescope, and was adopted by Sir William Herschel, who in
his grand speculations on the universe considered the nebulae as
immense masses of phosphorescent vapors, gradually condensing
around one or several centres into stars or clusters of stars. The
evidence afforded by the spectroscope seems to be in favor of such
an hypothesis, and shows us that gaseous agglomerations exist in
space.

ASTRONOMICAL DRAWINGS. 157

According to our modern conception, the visible universe is but
an infinitely small portion of the infinite universe perceived by our
mind. The great blazing centre around which our little, non-lumin-
ous globe pursues its endless journey, is only an humble member of
a cluster comprising four hundred equally powerful suns, as they
are believed to be, although they appear to us as little twinkling
stars. The nearest of these stars is 221,000 times as far from the
Sun as the Sun is from the Earth, and yet this entire cluster is only
one among the several hundred Star-clusters composing the great
galactic nebula in which we are involved, comprising thirty or fifty
millions of such suns. Among the 4,000 irresolvable nebulae in the
sky, perhaps over one-half are supposed to be galaxies, like our
own galaxy, composed of star-clusters, and millions of stars. Be-
sides these remote galaxies, vast agglomerations of yet uncon-
densed, nebulous matter exist in space, and form the nebulae proper,
in which the genesis of suns is slowly elaborated. Although the
visible universe is limited by the penetrating power of our instru-
ments, yet we see in imagination the infinite universe stretching
farther and farther; but we know not whether this invisible universe
is totally devoid of matter, or whether it also is filled with millions
and millions of suns and galaxies.

APPENDIX.

KEY TO THE PLATES.

{The numerals in brackets refer to pages of the MANUAL. The letters a, b, c, designate respectively
the upper, middle and lower portions of the page.]

PLATE I.— GKOUP OF SUN-SPOTS AND VEILED SPOTS.

Observed June 17, 1875, at 7h. 30m. A. M.

The background shows the sun's visible surface, or photosphere [26], as seen with a tele-
scope of high power at the most favorable moments, composed of innumerable light markings,
or granules [3c], separated by a network of darker gray. The granules, each some hun-
dreds of miles in width [4a], are thought to be the flame-like [5a] summits of the radial fila-
ments or columns of gas and vapor [5c] which compose the photospheric shell [10a, 17a]. The
two principal sun-spots [8a] of the group [156] here represented show the characteristic dark
umbra in the centre [96], overhung by the thatch-like penumbra [105], composed of whit-
ish gray filaments. The penumbral filaments are not supposed to differ in their nature from
those constituting the ordinary photosphere, save that they are seen here elongated and
violently disturbed by the force of gaseous currents [106]. Both spots are traversed
partly or wholly by bright overlying faculce [6a], or so-called luminous bridges [lOc], depressed
portions of which, in the left-hand spot, form the gray and rosy veils [9r] commonly
attendant upon this class of spots [13c]. In each of these spots, also, the inner ends of pro-
jecting penumbral filaments have fallen so far within the umbra [lla] as to appear much
darker than the rest. At the right of the upper portion of the left-hand spot, is a mass
of white facular clouds [146], honey-combed by dark spaces, through which are seen traces
of the undeveloped third spot of the triple group first observed [176]. If seen upon the sun's
limb, this would have presented the appearance of a lateral spot [136]. Above the right-hand
spot is a small black " dot," or incipient spot, without distinct penumbra [96]. The ir-
regular dark rift below the two large spots and connecting them is a spot of the crevasse type
[14a], with very slight umbra, a still better example of which is seen in a westward [vi. c] pro-
longation of the penumbra of the left-hand spot. In the upper left-hand corner of the Plate
are see*n several small faculce [6a], appearing as irregular whitish streaks amongst the gran-
ules [7c]. In the pear-shaped darkening of the solar surface below and at their left, is seen
A veue$spot [IGc], two of which attended this group [176].

Approximate scale, 2500 mUes—l inch.

160 APPENDIX.

PLATE II.— SOLAR PROTUBERANCES.
Observed May 5, 1873, at 9h. 40m. A. M.

A view of an upheaval of the chromosphere [18a], or third outlying envelope of tin- sun
[3a], as observed with the tele-spectroscope, or telescope with spectroscope attached.

The method of the observation requires a word of explanation. Save on the rare occasions
of a total solar eclipse, no direct telescopic view of the solar prominences or flames is possi-
ble, owing to the fact that the intense white light from the sun's main disk entirely ob-
scures [3a] the feeble pink light of the chromosphere [26cr]. A few years ago Messrs.
Jannsen and Lockyer [18&] found that a spectroscope of high dispersive power so weakens
the spectrum of ordinary sun-light as to show the spectrum of bright lines given by the
chromosphere [21a], on any clear day. The telescope is adjusted so that a portion of the
sun's limb, usually near a group of active sun-spots [21a], shall be presented before the
opened slit of the spectroscope. The light of the chromosphere thus admitted along with
some diffused sun-light from the earth's atmosphere, produces a spectrum of intensely bright
lines, widely separated, on the fainter background of the strongly dispersed spectrum of
sun-light. The most prominent of these bright lines are those known as the C line (scarlet),
Fliue (blue), which with several others are due to the hydrogen present in the chromosphere,
the Ds line (orange) ascribed to a little known substance called "helium" [5a], and occasion-
ally the sodium lines Di, Do, (yellow). By adjusting the slit upon the scarlet C line, the ap-
pearances represented in Plate II. were observed as through an atmosphere of scarlet light;
in the D or F lines identical appearances may be seen, but somewhat less clearly denned, as
through yellow or blue light respectively. Hence the solar flames, as here observed with
the spectroscope in the hydrogen C line, are seen through a portion only (the scarlet rays) of
the light coming from but one substance (hydrogen) of the companion incandescent sub-
stances present in the chromosphere. The color of the collective chromospheric light is seen
directly with the telescope during an eclipse (See Plate III.) to be a delicate rosy pink [26a.J

Description of the Plate. — The black background represents the general darkness of the
eye-piece to the spectroscope. * The broad red stripe stretching from top to bottom of the
Plate is a portion of the red band of the spectrum, magnified about 100 times as compared
with the actual spectroscopic view. The upper and lower edges of the cross-section of dnsky
red correspond with the edges of the slit, opened widely enough to admit a view of the
chromospheric crest and of the whole height of the protuberances at once. With a narrower
opening of the slit this background would have been nearly black, its reddish cast increasing
with the amount of opening and consequent admission of diffused sun-light. Rising above
the lower edge of the opening is seen a small outer segment of the chromosphere, which, as a
portion of the sun's eastern limb, should be imagined as moving directly towards the be-
holder. The seams and rifts by which its surface is broken, as well as the distorted forms
of the huge protuberances show the chromosphere to be in violent agitation. Some of the
most characteristic shapes [J9c] of the eruptive protuberances [20a] are presented, as also
cloud-like forms overtopping the rest. In the immediate foreground the bases of two tower-
ing columns appear deeply depressed below the general horizon of the segment observed,
showing an extraordinary velocity of motion of the whole uplifted mass toward the observer
[21c]. The highest of these protuberances was 126,000 miles in height at the moment of
observation [22a]. The triple protuberance at the left with two drooping wings [21b] and a
tall swaying spire tipped with a very bright flame, shows by its more brilliant color the higher
temperature (and possibly compression) to which its gases have been subjected [18c]. The

KEY TO THE PLATES. 161

irregular black bauds behind this protuberance indicate the presence there of less con-
densed and cooler clouds of the same gases [20c]. The dimmer jets of flame rising from
the chromosphere are either vanishing protuberances [20c], or, as in the case of the smallest
jet shown at the extreme right of the horizon, are the tops of protuberances just coming
into view.

Approximate scale, 6000 miles— 1 inch.

PLATE III. -TOTAL ECLIPSE OF THE SUN.

Observed July 29, 1878, at Oreiton, Wy.miiny Territory.

A telescopic view of the sun's corona or extreme outer atmosphere [36] and of the solar
flames or prominences [25c] during a total eclipse [23c]. At the moment of observation the
dark disk of the moon, while still hiding the sun's main body, had passed far enough east-
ward to allow the rosy pink chromospheric prominences to be seen on its western border
[2tic]. On all sides of the sun's hidden disk, the corona [256] shows its pale greenish light
[26a] extending in halo-like rays and streamers, and two very remarkable wings [266] stretch
eastward and westward very nearly in the plane of the ecliptic and in the direction of the
positions of Mercury and Venus respectively at the time of observation. The full extent of
these wings could not be shown in the Plate without reducing its scale materially, since the
westerly wing extended no less than twelve times the sun's diameter [276], and the easterly
wing nearly as far, or over ten million miles. A circlet of bright light immediately border-
ing the moon's disk is the so-called inner corona, next to which the wings and streamers are
brightest, thence shading off imperceptibly into the twilight sky of the eclipse [256]. Other
noteworthy peculiarities of the corona, as observed during this ealipse [265], are the varying
angles at which the radiating streamers are seen to project, the comparatively dark intervals
between them, and the curved, wisp-like projections seen upon the wings. An especially
noticeable gap appears where the most westerly of the upward streamers abruptly cuts off
the view of the long wing. The largest and brightest of the curving streamers on the west-
erly wing coincides with the highest flame-like protuberance [266]. To some observers
of this eclipse the upward and downward streamers seemed pointed at their outer extremi-
ties and less regular in form.

Approximate scale, 135,000 miles— I inch.

PLATE IV.— AURORA BOREALIS.
As observed March 1, 1872, at 9/t. 25m. P. M.

The view presents the rare spectacle of an aurora spanning the sky from east to west
in concentric arches [28c]. The Polar Star is nearly central in the back-ground, the constel-
lation of the Great Bear on the right and Cassiopeia's chair on the left. The large star at
some distance above the horizon on the right is Arcturus. The almost black inner segment
[286] of the aurora resting upon the horizon, has its summit in the magnetic meridian [32a] ,
which was in this case a little west of north, its arc being indented by the bases of the ascend-
ing streamers [28c]. Both streamers and arches were, when observed, tremulous with up-
ward pulsations [296] and there was also a wave-like movement of the streamers from west
to east [29a]. The prevailing color of this aurora is a pale whitish green [28c] and the com-
plementary red appears especially at the west end of the auroral arch. The summits of the
streamers are from four hundred to five hundred miles above the earth [31«] and the aurora
is therefore a phenomenon of the terrestrial atmosphere [326] rather than of astronomical
observation proper.

162 APPENDIX.

PLATE V.— THE ZODIACAL LIGHT.

Observed February 20, 1876.

An observation of the cone of light whose axis lies along the Zodiac [36&], whence it
derives its name. It is drawn as seen in the west [41a], with its base in the constellation
Pisces, and its apex near the familiar group of the Pleiades in the constellation Taurus. The
first bright star above the horizon in the base of the cone is the planet Venus and at some
distance above is the reddish disk of Mars, the two being in rare companionship as evening
stars. Above the constellation Pisces, two bright stars of Aries lie just outside the cone at
the right. The nearest bright star above these at the right is Beta, the leading star of the
constellation Triangula. Further at the right the three prominent stars nearly in a line are,
in ascending order, Delia, Beta and Gamma of the constellation Andromeda. Above these
at the left, the brightest star of a quadrangular group of four is the remarkable variable star
Algol (Beta) of the constellation Perseus, which changes from the second to the fourth mag-
nitude in a period of less than three days. At the left and a little above the Pleiades is the
ruddy star Aldebaran, one of the Hyades and chief star in the constellation Taurus. These
are the principal stars visible in this portion of the sky at the time of the observation. Their
relative positions are represented as seen in the sky and not by the common method of star-
atlases, which allows for the change from a spherical to a plane surface. Their magnitude in
the order of brightness is indicated only approximately [41a].

PLATE VI.— MAKE HUMORUM.

From a study made in 1875.

A view of one of the lunar plains [47c], or so-called seas (Maria), with an encircling
mountainous wall [45a] consisting of volcano-like craters [45c, 46c] in various stages of sub-
sidence and dislocation [48a]. The sun-light coming from the west casts strong shadows
from all the elevations eastward, and is just rising on the terminator [44c], where the rugged
structure of the Moon's surface is best seen. The lighter portions are the more elevated
mountainous tracts and crater summits [45a]. The detailed description of this Plate
given in the body of the MANUAL [51-2] is repeated here for convenience of reference: The
Mare Humorum, or sea of moisture, as it is called, is one of the smaller gray lunar
plains. Its diameter, which is very nearly the same in all directions, is about 270 miles, the
total area of this plain being about 50,000 square miles. It is one of the most distinct plains of
the Moon, and is easily seen with the naked eye on the left-hand side of the disk. The floor
of the plain is, like that of the other gray plains, traversed by several systems of very ex-
tended but low hills and ridges, while small craters are disseminated upon its surface.
The color of this formation is of a dusky greenish gray along the border, while in the in-
terior it is of a lighter shade, and is of brownish olivaceous tint. This plain, which is sur-
rounded by high clefts and rifts, well illustrates the phenomena of dislocation and subsidence.
The double-ringed crater Vitello, whose walls rise from 4,000 to 5,000 feet in height, is seen
in the upper left-hand corner of the gray plain. Close to Vitello at the east is the large
broken ring-plain Lee, and farther east, and a little below, is a similarly broken crater
called Doppelmayer. Both of these open craters have mountainous masses and peaks on
their floor, which is on a level with that of the Mare Humorum. A little below, and to
the left of these objects, is dimly seen a deeply imbedded oval crater, whose walls barely
rise above the level of the plain. On the right-hand side of the great plain is a long fault,
with a system of iracture running along its border. On this right-hand side may be seen

KEY TO THE PLATES. 163

a part of the line of the terminator, which separates the light from the darkness. Towards
the lower right-hand corner is the great ring-plain Gassendi, 55 miles in diameter, with its
system of fractures and its central mountains, which rise from 3,000 to 4,000 feet above its
floor. This crater slopes towards the plain, showing the subsidence to which it has been
submitted. While the northern portion of the wall of this crater rises to 10,000 feet, that on
the plain is only 500 feet high, and is even wholly demolished at one place where the floor
of the crater is in direct communication witlf the plain. In the lower part of the sea, and a
little to the west of the middle line, is found the crater Agatharchides, which shows below its
north wall the marks of rills impressed by a flood of lava, which once issued from the side of
the crater. On the left-hand side of the plain is seen the half-demolished crater Hippalus
resembling a large bay, which has its interior strewn with peaks and mountains. On this
same side can be seen one of the most important systems of clefts and fractures visible on
the Moon, these clefts varying in length from 150 to 200 miles.
Approximate scale, 15 miles— 1 inch.

PLATE VII.— PARTIAL ECLIPSE OF THE MOON.
Observed October 24, 1874.

A view of the Moon partially obscured by the Earth's shadow [53a], whose outline gives
ocular proof of the earth's rotundity of form. The shadowed part of the Moon's surface is
rendered visible by the diffused sun-light refracted upon it from the earth's atmosphere
[55a]. Its reddish brown color is due to the absorption, by vapors present near the earth's
surface [55a], of a considerable part of this dim light. On both the obscured and illu-
minated tracts the configurations of the Moon's surface are seen as with the naked eye
[566]. The craters [45c] appear as distinct patches of lighter color, and the noticeably
darker areas are the depressed plains or Maria [47c]. The large crater Tycho, at the lower
part of the disk, is the most prominent of these objects, with its extensive system of radiating
streaks [49a]. The largest crater above is Copernicus, at the left of which is Kepler and still
above are Arisiarchus and Herodotus appearing as if blended in one. Above and at the left
of the great crater Tycho, the first dark tract is the Mare Humorum of Plate VI., seen in its
natural position [vi.c], with the crater Gassendi at its northern (upper) extremity and Vitetto
on its southern (lower) border [516]. The advancing border of the shadow appears, as
always, noticeably darker than the remainder, an effect probably of contrast. The illu-
minated segment of the Moon's disk has its usual appearance, the lighter portions being the
more elevated mountainous surfaces, and the dark spaces the floors of extensive plains.

Approximate scale, 140 mues—l inch.

PLATE VIII.— THE PLANET MAES.
Observed September 3, 1877, at llh. 55m. P. M.

A view of the southern hemisphere of Mars [64a], when in the most favorable position for
observation [616], and when exceptionally free from the clouds, which very frequently hide its
surface configurations [636]. Since, of all the planets, Mars is most like the earth [71c],
Plate VIII. may give a fair idea of the appearance of our globe to a supposed observer on Mars.
The dark gray and black markings [63c], are regarded as tracts of water [656], or of some
liquid with similar powers of absorbing light; and, for the same reason, the lighter portions,
of a prevailing reddish tint [70c], are supposed to be bodies of land [656], while the bright
white portions are variously due to clouds [686], to polar snow or ice [656, 676], and the bright
rim of white along the limb, to the depth of the atmosphere through which the limb is seen

164 APPENDIX.

£70c]. The chief permanent features of the planet's surface have been named in honor of
various astronomers.

The large dark tract on the left is De La Rue Ocean, the isolated oval spot near the centre is
Terby Sea, and on the right is the western end of Maraldi Sea, with strongly indented border.
Directly north of (below) De La Eue Ocean, is Maedler Continent; above it stretches Jacob Land;
and surrounding Terby Sea is Secchi Continent. Extending into the centre of De La Hue Ocean is
a curious double peninsula, called, in consequence of the dimness of former observations,
Hall Island [69a]. The sharply defined, white-crested northern borders of De La Rue Ocean
and Maraldi Sea may indicate the existence there of lofty coast ranges, more or less con-
stantly covered with opaque clouds [636] strongly reflecting light [656]. The white spot in
the centre of Maedler Continent, of a temporary nature, has a similar explanation [69c]. The
intervals of olivaceous gray on Secchi Continent and elsewhere may perhaps be ascribed [65c]
to the flooding and drying up of marshes and lowlands, as these markings have been ob-
served to vary somewhat in connection with the change of seasons on Mars [71c]. The green-
ish tints observed along the planet's limb, alike on the darker and lighter surfaces, are prob-
ably due to an optical effect, the green being complementary to the prevailing red of the disk.
The brilliant oval white spot near the southern (upper) pole of the planet is a so-called polar
spot [65-67], in all probability consisting of a material similar to snow or ice [676] and here
observed in the midst of a dark open sea [67c].

Approximate scale, 300 mites— 1 inch.

PLATE IX.— THE PLANET JUPITEK.

Observed November 1, 1880, at 9ft. 30m. P. M.

This planet is perpetually wrapped in dense clouds [756] which hide its inner globe from
view. The drawing shows Jupiter's outer clouded surface with its usual series of alternate
light and dark belts [746], the disk as a whole appearing brighter in the centre than near the
limb. The darker gray and black markings [74c] indicate in general the lower cloud-levels ;
that is, partial breaks or rifts in the cloudy envelope, whose prevailing depth apparently
exceeds four thousand miles [79c-80c]. While the deepest depression in the cloudy envelope
is within the limits of the Great Bed Spot [77a], the vision may not even here penetrate
very deeply. Two of Jupiter's four moons [77c] present bright disks [78c] near the
planet's western limb, and cast their shadows [79a] far eastward on the disk, that of the
" second satellite •' falling upon the Bed Spot [82a]. On the Bed Spot are seen in addition
two small black spots [77a], no explanation of which can yet be offered. The broad white
ring of clouds bordering the Bed Spot [776] appeared in constant motion. The central, or
equatorial belt [74c], shows brilliant cloudy masses of both the cumulus and stratus types, and
the underlying gray and black cloudy surfaces are pervaded with the pinkish color charac-
teristic of this belt. The dark circular spots on the wide white belt next north showed in their
mode of formation [76c] striking resemblances to sun-spots [82a]. They afterward coalesced
into a continuous pink belt. The diffusion of pinkish color over the three northernmost dark
bands, as here observed, is unusual [75a]. About either pole is seen the uniform gray seg-
ment [75a] or polar cap. The equatorial diameter is noticeably longer [73c] than the polar
diameter, a consequence of the planet's extraordinary swiftness of rotation. To tho same
cause may also be due chiefly the distribution of the cloudy belts parallel to the planet's
equator [746], though the analogy of the terrestrial trade-winds fails to explain all the ob-
served phenomena.

Approximate scale, 5,500 miles— 1 inch.

KEF TO THE PLATES. 165

PLATE X.— THE PLANET SATUKN.

Observed November 30, 1874, at 5h. 50m. P. M.

Saturn is unique amongst the planets in that its globe is encircled by a series of concen-
tric rings [86c], which lie in the plane of its equator, and consist, according to present theories,
of vast throngs of minute bodies revolving about the planet, like so many satellites, in closely
parallel orbits [97c]. The globe of Saturn, like that of Jupiter, is surrounded by cloudy belts
parallel to its equator [85c]. The broad equatorial belt, of a delicate pinkish tint, is both
brighter and more mottled [85c] than the narrower yellowish white belts, which alternate with
darker belts of ashy gray [86a] on both the north and south sides, but are seen here only on
the northern side. The disk has an oval shape, owing to the extreme polar compression of
the globe [856].

The outer, middle and inner rings, with their various subdivisions [86c], are clearly shown
in Plate X., and are best seen on the so-called ansce, or handles, projecting on either side.
The gray outer ring is separated by the dusky pencil line [86c] into two divisions, both of which
appear slightly mottled on the ansae, as if with clouds [87a]. The middle ring has three sub-
divisions which are clearly distinguishable, although separated by no dark interval, viz., a
brilliant white outer zone, distinctly mottled, as seen on the extremities of the ansae, and two
interior zones of gradually diminishing brightness [87a]. The gauze or dusky ring is seen at
its full width on the ansae, but on the background of the strongly illuminated globe only its
outer and presumably denser border [87c] is visible. The sha'dow of the globe on the rings
[886] is seen on the lower portion of the eastern ansa. The shadow on the dusky ring
[88c] is with difficulty perceptible; the shadow on the middle ring is slightly concave toward
the planet, which concavity is abruptly increased on the outer zone of this ring [89a] ; while
the shadow on the outer ring slants away from the globe. These appearances are fully
accounted for by supposing a general increase of level from the inner edge of the dusky ring
to the outer margin of the middle ring, and a uniform lower level on the outer ring. Other
observers have regarded the deflection of the shadow as an effect of irradiation [89-90].
The inner margin of the double outer ring presents on both ansse a number of slight
indentations, which, if not actual irregularities in the contour of this ring, may be explained
as shadows caused by elevations on the outer border of the middle ring, or possibly by over-
hanging clouds.

Approximate scale, 6,500 miles— I inch.

PLATE XI.— THE GEE AT COMET OF 1881.
Observed on the night of June 25-26, at Ih. 30m. A. M.

A view of the comet 1881, III., drawn as if seen with the naked eye, the minute details,
however, being reproduced as seen with the telescope [vi.c]. The star-like nucleus [1036] is
attended by four conical wings [104a] which cause it to appear diamond-shaped. The coma
[1056] appears double, the brilliant inner coma, immediately enveloping the nucleus, being
surrounded by a fainter exterior coma [106a], which has a noticeable depression correspond-
ing to that of the inner edge of the principal coma. The tail is divided lengthwise by a dark
rift [1066] and is brightest on its convex or forward side [106c]. An inner portion of the tail,
brighter than the rest, is more strongly curved, as if by solar repulsion [105c, 113]. Stars, are
seen through the brighter parts of the tail, as they may be seen even through the coma and
nucleus, with little diminution of their light [1116].

1G6 APPENDIX.

PLATE XII.-THE NOVEMBEK METEORS.

As observed on the night of November 13-14, 1868.

A partially ideal view of the November Meteors [116c], combining forms observed
[vii.a] at different times during the night of Nov. 13th, 1868. It is not, however, a fanciful
view [118c], since a much larger number of meteors were observed falling at once during the
shower of November, 1833, and at other times [1166]. The locality of the observation is shown
by the Polar Star seen near the centre of the Plate, and Cassiopeia's Chair at the left. The
general direction of the paths of the meteors is from the north-east, the radiant point [120a]
of the shower having been in the constellation Leo [121a], beyond and above Ursa Major.
While the orbits of the meteors are, in general, curved regularly and slightly, [117a], several
are shown with very eccentric paths [1176], among them one which changed its course at a sharp
angle. In the upper left-hand corner appear two vanishing trails of the " ring-form," and sev-
eral others still further transformed into faint luminous patches of cloud [1186]. Bed, yellow,
green, blue and purple tints were observed in the meteors and their trails [1186], as repre-
sented in the Plate.

PLATE XIII.— PART OF THE MILKY WAY.

From a study made during the years 1874, 1875 and 1876.

The course of the portion of the Galaxy [130] represented in Plate XIII. is as follows :
From Cassio.peia's chair, three bright stars of which appear at the upper edge of the Plate,
the Galaxy, forming two streams, descends south, passing partly through Lacerta on the left,
and Cepheus on the right ; at this last point it approaches nearest to the Polar Star, which is
itself outside of the field of view. Then it enters Cygnus, where it becomes very complicated
and bright, and where several large cloudy masses are seen terminating its left branch, which
passes to the right, near the bright star Deneb, the leader of this constellation. Below Deneb,
the Galaxy is apparently disconnected and separated from the northern part by a narrow,
irregular dark gap. From this rupture, the Milky-way divides into two great streams, sepa-
rated by an irregular dark rift. An immense branch extends to the right, which, after having
formed an important luminous mass between the stars Gamma and Beta, continues its south-
ward progress through parts of Lyra, Vulpecula, Hercules, Aquila and Ophiuchus, where it
gradually terminates a few degrees south of the equator. The main stream on the left, after
having formed a bright mass around Epsuon Oygni, passes through Vulpecula and then
Aquila, where it crosses the equinoctial just below the star Eta., after having involved in its
nebulosity the bright star AUair, the leader of Aquila. In the southern hemisphere the
Galaxy becomes very complicated and forms a succession of very bright, irregular masses,
the upper one being in Scutum Sobieskii, while the others are respectively situated in Sagit-
tarius and in Scorpio ; the last, just a little above our horizon, being always considerably
dimmed by vapors. From Scutum Sobieskii, the Galaxy expands considerably on the right,
and sends a branch into Scorpio, in which the fiery red star Antares is somewhat involved.
In the upper left-hand corner of the Plate, at some distance from the Milky-way, is seen
dimly the Nebula in Andromeda, which becomes so magnificent an object to telescopic view

PLATE XIV. -STAR-CLUSTER IN HERCULES.

From a study made in June, 1877.

In the constellation Hercules [137c], a small nebulous mass is faintly visible to the eye
[1386], a telescopic view of which is presented in Plate XIV. It is one of the most beautiful

KEY TO THE PLATES. 167

of the easily resolvable [139a] globular clusters [1396]. The brilliancy of the centre gives
the cluster a distinctly globular appearance, while the several wings [1406] curving in vari-
ous directions, have suggested to some observers an irregularly spiral structure [140c]. The
large stars of the cluster are arranged in several groups which correspond, in a general way,
with the faintly luminous wings.

PLATE XV.— THE GREAT NEBULA IN ORION.

From a study made in the years 1875-76.

This nebula, which is one of the most brilliant and wonderful of telescopic objects,
readily visible to the naked eye as a patch of nebulous light immediately surrounding the
middle star of the three which form the sword of Orion, and a little south of the three well-
known stars forming the belt [153a]. The small stars in this, as in other Plates of the series,
are somewhat exaggerated in size, as was unavoidable with any mode of reproduction that
could be employed [41o]. The bright pentagonal centre of the nebula [1526] is traversed by
less luminous rifts, the several subdivisions thus outlined being irregularly mottled as if
by bright fleecy clouds [154a]. Toward the lower part of this bright pentagonal centre is a
comparatively dark space containing four bright stars which form a trapezium and together
constitute the quadruple star Thela Orionis, which, to the naked eye, appears as the single
star in the centre of the sword. On three sides of the central mass extend long bright wisps,
whose curves fail, however, to reveal the spiral structure often attributed to this nebula
[1546]. On the east a broad wing, with wave-shaped inner border, stretches southward
[1536]. East of the trapezium are two especially noticeable dark spaces. Close to the main
nebula on the north-east, a small faint nebula surrounds a bright star, and a branch from
another faint stream of nebulous matter forming a loop to the southward, encloses the nebu-
lous star (Iota Orionis) shown at the top of the Plate.

UNIVERSITY OF CALIFORNIA LIBRARY
BERKELEY

Return to desk from which borrowed.
This book is DUE on the last date stamped below.

LIBFIARY

LD 21-100TO-ll,'49(B7146sl6)476

670903

UNIVERSITY OF CALIFORNIA LIBRARY